Implementation of async-await transformation on js ast.

pkg/compiler/lib/src/js/rewrite_async.dart

Index: pkg/compiler/lib/src/js/rewrite_async.dart
diff --git a/pkg/compiler/lib/src/js/rewrite_async.dart b/pkg/compiler/lib/src/js/rewrite_async.dart
new file mode 100644
index 0000000000000000000000000000000000000000..89762dd82359129508727f8b2d3b0bd788195b75
--- /dev/null
+++ b/pkg/compiler/lib/src/js/rewrite_async.dart
@@ -0,0 +1,2161 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library rewrite_async;
+
+// TODO(sigurdm): Avoid using variables in templates. It could blow up memory
+// use.
+
+import "dart:math" show max;
+import 'dart:collection';
+
+import "js.dart" show
+    ArrayHole,
+    ArrayInitializer,
+    Assignment,
+    AsyncModifier,
+    Await,
+    Binary,
+    Blob,
+    Block,
+    Break,
+    Call,
+    Case,
+    Catch,
+    Comment,
+    Conditional,
+    Continue,
+    DartYield,
+    Default,
+    Do,
+    EmptyStatement,
+    Expression,
+    ExpressionStatement,
+    For,
+    ForIn,
+    Fun,
+    FunctionDeclaration,
+    If,
+    InterpolatedExpression,
+    InterpolatedLiteral,
+    InterpolatedParameter,
+    InterpolatedSelector,
+    InterpolatedStatement,
+    LabeledStatement,
+    Literal,
+    LiteralBool,
+    LiteralExpression,
+    LiteralNull,
+    LiteralNumber,
+    LiteralStatement,
+    LiteralString,
+    Loop,
+    NamedFunction,
+    New,
+    Node,
+    NodeVisitor,
+    ObjectInitializer,
+    Parameter,
+    Postfix,
+    Prefix,
+    Program,
+    Property,
+    PropertyAccess,
+    RegExpLiteral,
+    Return,
+    Statement,
+    Switch,
+    SwitchClause,
+    This,
+    Throw,
+    Try,
+    VariableDeclaration,
+    VariableDeclarationList,
+    VariableInitialization,
+    VariableUse,
+    While,
+    js,
+    number;
+
+import '../util/util.dart';
+import '../dart2jslib.dart' show DiagnosticListener;
+
+import "../helpers/helpers.dart";
+
+/// Rewrites a js Fun with async/sync*/async* functions and await and yield
+/// (with dart-like semantics) to an equivalent function without these.
+/// await-for is not handled and must be rewritten before. (Currently handled
+/// in ssa/builder.dart).
+///
+/// When generating the input to this, special care must be taken that
+/// parameters to sync* functions that are mutated in the body must be boxed.
+/// (Currently handled in closure.dart).
+///
+/// Look at [visitFun], [visitDartYield] and [visitAwait] for more explanation.
+class AsyncRewriter extends NodeVisitor {
+
+  // Local variables are hoisted to the top of the function, so they are
+  // collected here.
+  List<VariableDeclaration> localVariables = new List<VariableDeclaration>();
+
+  Map<Node, int> continueLabels = new Map<Node, int>();
+  Map<Node, int> breakLabels = new Map<Node, int>();
+  Map<Node, int> finallyLabels = new Map<Node, int>();
+  int returnLabel;
+
+  // A stack of all enclosing jump targets (including the function for
+  // representing the target of a return, and all enclosing try-blocks that have
+  // finally part, this way ensuring all the finally blocks between a jump and
+  // its target are run before the jump.
+  List<Node> targetsAndTries = new List<Node>();
+
+  List<int> continueStack = new List<int>();
+  List<int> breakStack = new List<int>();
+  List<int> returnStack = new List<int>();
+
+  List<Pair<String, String>> variableRenamings =
+      new List<Pair<String, String>>();
+
+  PreTranslationAnalysis analysis;
+
+  List<int> errorHandlerLabels = new List<int>();
+
+  final Function safeVariableName;
+
+  // All the <x>Name variables are names of Javascript variables used in the
+  // transformed code.
+
+  /// Contains the result of an awaited expression, or a conditional or
+  /// lazy boolean operator.
+  ///
+  /// It is a parameter to the [helperName] function.

[floitsch, 2015/02/05 20:17:22]

Doesn't really help in understanding what it's for.

If it really is only for the three cases (awaited expression, conditional or
lazy boolean operator), maybe give examples?

Why is it a parameter to the helper?

[sigurdm, 2015/02/06 14:26:32]

Tried to give an example.

[floitsch, 2015/02/06 14:58:43]

Never mind.
I was again confused by the similarity in names of "thenHelper" and
"helperName".

[sigurdm, 2015/02/06 15:21:20]

Acknowledged.

+  String resultName;

[floitsch, 2015/02/05 20:17:22]

Future CL:

consider:
 js.Expression result = uncachedExpressionTemplate(resultName);

Basically these could be the only JS expressions that are uncached, and then
it's not the "name" anymore, either.

[sigurdm, 2015/02/06 14:26:33]

I started doing something like this, but one problem is that the js template
system does not allow holes in declaration positions, so I cannot have:

js("function #helper() {
  ...
}")

+
+  /// The name of the inner function that is scheduled to do each await/yield,
+  /// and called to do a new iteation for sync*.

[floitsch, 2015/02/05 20:17:21]

iteration

[sigurdm, 2015/02/06 14:26:32]

Done.

+  String helperName;
+
+  /// The Completer that will finish an async function.

[floitsch, 2015/02/05 20:17:21]

What if we are in a sync* or async* ?

[sigurdm, 2015/02/06 14:26:33]

Done.

+  String completerName;
+
+  /// The StreamController that controls an async* function.

[floitsch, 2015/02/05 20:17:21]

What if we are in a sync* or async function?

[sigurdm, 2015/02/06 14:26:33]

Done.

+  String controllerName;
+
+
+  /// Used to simulate a goto.
+  ///
+  /// To "goto" a label, the label is assigned to this
+  /// variable, and break out of the switch to take another iteration in the
+  /// while loop. See [addGoto]
+  String gotoName;
+
+  /// The label of the current error handler.
+  String handlerName;
+
+  /// Current caught error.
+  String errorName;
+
+  /// A stack of labels of finally blocks to visit, and the label to go to after
+  /// the last.
+  String nextName;
+
+  /// The current returned value (a finally block may overwrite it).
+  String returnValueName;
+
+  /// The label of the outer loop.
+  ///
+  /// Used if there are untransformed loops containing break or continues to
+  /// targets outside the loop.
+  String outerLabelName;
+
+  /// If javascript `this` is used, it is accessed via this variable, in the
+  /// [helperName] function.
+  String selfName;
+
+  // These expressions are hooks for communicating with the runtime.
+
+  /// The function called by an async function to simulate an await or return.
+  ///
+  /// For an await it is called with:
+  ///
+  /// - The value to await
+  /// - The [helperName]
+  /// - The [completerName]
+  /// - A Javascript function that will take the program to the right error

[floitsch, 2015/02/05 20:17:22]

that takes...

It's not just taking the program to the right error-handler. It is executing the
helper.

Maybe just "A JavaScript function that is executed if the future completed with
an error". That function is responsible for executing the right error handler
(and/or finallies).

[sigurdm, 2015/02/06 14:26:32]

Done.

+  /// handler in case the future completes with an error.
+  ///
+  /// For a return it is called with:
+  ///
+  /// - The value to complete the completer with.
+  /// - null
+  /// - The [completerName]
+  /// - null.
+  final Expression thenHelper;
+
+  /// The function called by an async* function to simulate an await, yield or
+  /// yield*.
+  ///
+  /// For an await/yield/yield* it is called with:
+  ///
+  /// - The value to await/yieldExpression(value to yield)/
+  /// yieldStarExpression(stream to yield)
+  /// - The [helperName]
+  /// - The [completerName]

[floitsch, 2015/02/05 20:17:20]

controllerName ?

[sigurdm, 2015/02/06 14:26:33]

Done.

+  /// - A Javascript function that will take the program to the right error

[floitsch, 2015/02/05 20:17:21]

ditto.

[sigurdm, 2015/02/06 14:26:32]

Done.

+  /// handler in case the future completes with an error.
+  ///
+  /// For a return it is called with:
+  ///
+  /// - null
+  /// - null
+  /// - The [controllerName]
+  /// - null.
+  final Expression streamHelper;
+
+  /// How to initialize the [completerName] variable.

[floitsch, 2015/02/05 20:17:22]

It's not a "how".

The constructor that is used to initialize the [completerName] variable.

[sigurdm, 2015/02/06 14:26:32]

Done.

+  ///
+  /// Specific to async methods.
+  final Expression newCompleter;
+
+  /// How to initialize the [controllerName] variable.

[floitsch, 2015/02/05 20:17:21]

ditto.

[sigurdm, 2015/02/06 14:26:32]

Done.

+  ///
+  /// Specific to async* methods.
+  final Expression newController;

[floitsch, 2015/02/05 20:17:21]

It's inconsistent that "newCompleter" and newIterable are constructors but
"newController" apparently is a static function.

[sigurdm, 2015/02/06 14:26:33]

true. Made them all constructors.

+
+  /// Creates an Iterable for a sync* method. Called with [helperName]

[floitsch, 2015/02/05 20:17:22]

Missing trailing ".".

[sigurdm, 2015/02/06 14:26:33]

Done.

+  final Expression newIterable;
+
+  /// Creates a marker showing that iteration is over.

[floitsch, 2015/02/05 20:17:21]

A JS Expression that creates ...

[sigurdm, 2015/02/06 14:26:32]

Done.

+  ///
+  /// Called without arguments.
+  final Expression endOfIteration;
+
+  /// Creates a marker indicating a 'yield' statement.

[floitsch, 2015/02/05 20:17:21]

ditto.

[sigurdm, 2015/02/06 14:26:33]

Done.

+  ///
+  /// Called with the value to yield.
+  final Expression yieldExpression;
+
+  /// Creates a marker indication a 'yield*' statement.

[floitsch, 2015/02/05 20:17:21]

ditto.

[sigurdm, 2015/02/06 14:26:34]

Done.

+  ///
+  /// Called with the stream to yield from.
+  final Expression yieldStarExpression;
+
+  final DiagnosticListener diagnosticListener;
+  // For error reporting only.
+  Spannable get spannable {
+    return (_spannable == null) ? NO_LOCATION_SPANNABLE : _spannable;
+  }
+
+  Spannable _spannable;
+
+  int _currentLabel = 0;
+
+  // The highest temporary variable index currently in use.
+  int currentTempVarIndex = 0;
+  // The highest temporary variable index ever in use in this function.
+  int tempVarHighWaterMark = 0;
+  Map<int, Expression> tempVarNames = new Map<int, Expression>();
+
+  AsyncModifier async;
+
+  bool get isSync => async == const AsyncModifier.sync();
+  bool get isAsync => async == const AsyncModifier.async();
+  bool get isSyncStar => async == const AsyncModifier.syncStar();
+  bool get isAsyncStar => async == const AsyncModifier.asyncStar();
+
+  AsyncRewriter(this.diagnosticListener,
+                spannable,
+                {this.thenHelper,
+                 this.streamHelper,
+                 this.newCompleter,
+                 this.newController,
+                 this.endOfIteration,
+                 this.newIterable,
+                 this.yieldExpression,
+                 this.yieldStarExpression,
+                 this.safeVariableName})
+      : _spannable = spannable;
+
+  /// Main entry point.
+  /// Rewrites a sync*/async/async* function to an equivalent normal function.
+  ///
+  /// [spannable] can be passed to have a location for error messages.
+  Fun rewrite(Fun node, [Spannable spannable]) {
+    _spannable = spannable;
+
+    async = node.asyncModifier;
+    assert(!isSync);
+
+    analysis = new PreTranslationAnalysis(unsupported);
+    analysis.analyze(node);
+
+    // To avoid name collisions with existing names, the fresh names are
+    // generated after the analysis.
+    resultName = freshName("result");
+    completerName = freshName("completer");
+    controllerName = freshName("controller");
+    helperName = freshName("helper");
+    gotoName = freshName("goto");
+    handlerName = freshName("handler");
+    errorName = freshName("error");
+    nextName = freshName("next");
+    returnValueName = freshName("returnValue");
+    outerLabelName = freshName("outer");
+    selfName = freshName("self");
+
+    return node.accept(this);
+  }
+
+  Expression get currentErrorHandler {
+    return errorHandlerLabels.isEmpty ?
+        new LiteralNull() :
+        number(errorHandlerLabels.last);
+  }
+
+  int allocateTempVar() {
+    assert(tempVarHighWaterMark >= currentTempVarIndex);
+    currentTempVarIndex++;
+    tempVarHighWaterMark = max(currentTempVarIndex, tempVarHighWaterMark);
+    return currentTempVarIndex;
+  }
+
+  VariableUse useTempVar(int i) {
+    return tempVarNames.putIfAbsent(i, () =>
+        new VariableUse(freshName("temp$i")));
+  }
+
+  /// Generates a variable name with [safeVariableName] based on [originalName]
+  /// with a suffix to guarantee it does not collide with already used names.
+  String freshName(String originalName) {
+    String safeName = safeVariableName(originalName);
+    String result = safeName;
+    int counter = 1;
+    while (analysis.usedNames.contains(result)) {
+      result = "$safeName$counter";
+      ++counter;
+    }
+    analysis.usedNames.add(result);
+    return result;
+  }
+
+  /// All the pieces are collected in this map, to create a switch with a case
+  /// for each label.
+  ///
+  /// The order is important, therefore the type is explicitly LinkedHashMap.
+  LinkedHashMap<int, List<Statement>> labelledParts =
+      new LinkedHashMap<int, List<Statement>>();
+
+  /// Description of each label for readability of the non-minified output.
+  Map<int, String> labelComments = new Map<int, String>();
+
+  /// True if the function has any try blocks containing await.
+  bool hasTryBlocks = false;
+
+  /// True if any return, break or continue passes through a finally.
+  bool hasJumpThroughFinally = false;
+
+  /// True if the traversion currently is inside a loop or switch for which
+  /// [shouldTransform] is false.
+  bool insideUntranslatedBreakable = false;
+
+  /// True if a label is used to break to an outer switch-statement.
+  bool hasJumpThoughOuterLabel = false;
+
+  /// Buffer for collecting translated statements belonging to the same switch
+  /// case.
+  List<Statement> currentStatementBuffer;
+
+  // Labels will become cases in the big switch expression, and `goto label`
+  // is expressed by assigning to the switch key [gotoName] and breaking out of
+  // the switch.
+
+  int newLabel([String comment]) {
+    int result = _currentLabel;
+    _currentLabel++;
+    if (comment != null) {
+      labelComments[result] = comment;
+    }
+    return result;
+  }
+
+  /// Begins outputting statements to a new buffer with label [label].
+  ///
+  /// Each buffer ends up as its own case part in the big state-switch.
+  void beginLabel(int label) {
+    assert(!labelledParts.containsKey(label));
+    currentStatementBuffer = new List<Statement>();
+    labelledParts[label] = currentStatementBuffer;
+    addStatement(new Comment(labelComments[label]));
+  }
+
+  /// Returns a statement assigning to the variable named [gotoName].
+  /// This should be followed by a break for the goto to be executed. Use
+  /// [gotoWithBreak] or [addGoto] for this.
+  Statement setGotoVariable(int label) {
+    return new ExpressionStatement(
+        new Assignment(new VariableUse(gotoName), number(label)));
+  }
+
+  /// Return a block that has a goto to [label] including the break.

[floitsch, 2015/02/05 20:17:22]

Returns

[sigurdm, 2015/02/06 14:26:33]

Done.

+  ///
+  /// Also inserts a comment describing the label if available.
+  Block gotoAndBreak(int label) {
+    List<Statement> statements = new List<Statement>();
+    if (labelComments.containsKey(label)) {
+      statements.add(new Comment("goto ${labelComments[label]}"));
+    }
+    statements.add(setGotoVariable(label));
+    if (insideUntranslatedBreakable) {
+      hasJumpThoughOuterLabel = true;
+      statements.add(new Break(outerLabelName));
+    } else {
+      statements.add(new Break(null));
+    }
+    return new Block(statements);
+  }
+
+  /// Add a goto to [label] including the break.

[floitsch, 2015/02/05 20:17:22]

Adds

[sigurdm, 2015/02/06 14:26:33]

Done.

+  ///
+  /// Also inserts a comment describing the label if available.
+  void addGoto(int label) {
+    if (labelComments.containsKey(label)) {
+      addStatement(new Comment("goto ${labelComments[label]}"));
+    }
+    addStatement(setGotoVariable(label));
+
+    addBreak();
+  }
+
+  void addStatement(Statement node) {
+    currentStatementBuffer.add(node);
+  }
+
+  void addExpressionStatement(Expression node) {
+    addStatement(new ExpressionStatement(node));
+  }
+
+  /// True if there is an await or yield in [node] or some subexpression.
+  bool shouldTransform(Node node) {
+    return analysis.hasAwaitOrYield.contains(node);
+  }
+
+  void unsupported(Node node) {
+    throw new UnsupportedError(
+        "Node $node cannot be transformed by the await-sync transformer");
+  }
+
+  void unreachable(Node node) {
+    diagnosticListener.internalError(spannable,
+        "Internal error, trying to visit $node");
+  }
+
+  visitStatement(Statement node) {
+    node.accept(this);
+  }
+
+  /// Visits [node] to ensure its sideeffects are performed, but throwing away
+  /// the result.
+  ///
+  /// If the return value of visiting [node] is an expression guaranteed to have
+  /// no side effect, it is dropped.
+  void visitExpressionIgnoreResult(Expression node) {
+    Expression result = node.accept(this);
+    if (!(result is Literal || result is VariableUse)) {
+      addExpressionStatement(result);
+    }
+  }
+
+  Expression visitExpression(Expression node) {
+    return node.accept(this);
+  }
+
+  /// Emits an assignment of [result] to a temporary variable unless it can
+  /// be guaranteed that evaluating [result] has no side-effect, and has
+  /// constant value.
+  ///
+  /// Returns the temporary variable or the result itself.
+  ///
+  /// For example:
+  ///
+  /// - _storeIfNecessary(someLiteral) returns someLiteral.
+  /// - _storeIfNecessary(someVariable)
+  ///   inserts: var tempX = someVariable
+  ///   returns: tempX
+  ///   where tempX is a fresh temporary variable.
+  Expression _storeIfNecessary(Expression result) {
+    // Note that RegExes, ArrayInitializer and ObjectInitializer are not
+    // [Literal]s.
+    if (result is Literal) return result;
+    Expression tempVar = useTempVar(allocateTempVar());
+    addExpressionStatement(new Assignment(tempVar, result));
+    return tempVar;
+  }
+
+  withExpression(Expression node, fn(Expression result), {bool store}) {
+    int oldTempVarIndex = currentTempVarIndex;
+    Expression visited = visitExpression(node);
+    if (store) {
+      visited = _storeIfNecessary(visited);
+    }
+    var result = fn(visited);
+    currentTempVarIndex = oldTempVarIndex;
+    return result;
+  }
+
+  /// Calls [fn] with the value of evaluating [node1] and [node2].
+  ///
+  /// If `shouldTransform(node2)` the first expression is stored in a temporary
+  /// variable.
+  ///
+  /// This is because node1 must be evaluated before visiting node2,

[floitsch, 2015/02/05 20:17:21]

I'm not sure I fully understand the comment. My best attempt:

Calls [fn] with the value of evaluating [node1] and [node2].

Both nodes are evaluated in order.

If node2 must be transformed (see [shouldTransform]), then the evaluation of
node1 is added to the current statement-list and the result is stored in a
temporary variable. The evaluation of node2 is then free to emit statements
without affecting the result of node1.

This is necessary, because await or yield expressions have to emit statements,
and these statements could affect the value of node1.

[sigurdm, 2015/02/06 14:26:33]

Better. Thanks.

+  /// because the evaluation of an await or yield cannot be expressed as
+  /// an expression, visiting node2 it will output statements that
+  /// might have an influence on the value of node1.
+  withExpression2(Expression node1, Expression node2, fn(
+      Expression result1,
+      Expression result2)) {
+    int oldTempVarIndex = currentTempVarIndex;
+    Expression r1 = visitExpression(node1);
+    if (shouldTransform(node2)) {
+      r1 = _storeIfNecessary(r1);
+    }
+    Expression r2 = visitExpression(node2);
+    var result = fn(r1, r2);
+    currentTempVarIndex = oldTempVarIndex;
+    return result;
+  }
+
+  /// Calls [fn] with the value of evaluating all [nodes].
+  ///
+  /// All results before the last node where `shouldTransform(node)` are stored
+  /// in temporary variables.
+  ///
+  /// See more explanation on [withExpression2].
+  ///
+  /// If any of the nodes are null, they are ignored, and a null is passed to
+  /// [fn] in that place.
+  withExpressions(List<Expression> nodes, fn(List<Expression> results)) {
+    int oldTempVarIndex = currentTempVarIndex;
+    // Find last occurence of a 'transform' expression in [nodes].
+    // All expressions before that must be stored in temp-vars.
+    int lastTransformIndex = 0;
+    for (int i = nodes.length - 1; i >= 0; --i) {
+      if (nodes[i] == null) continue;
+      if (shouldTransform(nodes[i])) {
+        lastTransformIndex = i;
+        break;
+      }
+    }
+    List<Node> visited = nodes.take(lastTransformIndex).map((Node node) {
+      return node == null
+          ? null
+          : _storeIfNecessary(visitExpression(node));
+    }).toList();
+    visited.addAll(nodes.skip(lastTransformIndex).map((Node node) {
+      return node == null
+          ? null
+          : visitExpression(node);
+    }));
+    var result = fn(visited);
+    currentTempVarIndex = oldTempVarIndex;
+    return result;
+  }
+
+  /// Emits the return block that all returns should jump to (after going
+  /// through all the enclosing finally blocks) the jump to here is made in

[floitsch, 2015/02/05 20:17:22]

blocks). The jump ...

[sigurdm, 2015/02/06 14:26:32]

Done.

+  /// [visitReturn].
+  ///
+  /// Returning from an async method calls the [thenHelper] with the result.
+  /// (the result might have been stored in [returnValueName] by some finally
+  /// block).
+  ///
+  /// Returning from a sync* function returns an [endOfIteration] marker.
+  ///
+  /// Returning from an async* function calls the [streamHelper] with an
+  /// [endOfIteration] marker.
+  void addReturn() {

[floitsch, 2015/02/05 20:17:22]

addFooter ?
addExit ?
addFunctionExit ?

(`addReturn` is probably fine, but I have to think of a return statement, which
confused me last time).

[sigurdm, 2015/02/06 14:26:32]

I go with addExit.

+    if (analysis.hasExplicitReturns || isAsyncStar) {
+      beginLabel(returnLabel);
+    } else {
+      addStatement(new Comment("implicit return"));
+    }
+    switch (async) {
+      case const AsyncModifier.async():
+        String returnValue = analysis.hasExplicitReturns
+            ? returnValueName
+            : "null";
+        addStatement(js.statement(
+            "return #thenHelper($returnValue, null, $completerName, null)",
+            {"thenHelper": thenHelper}));
+        break;
+      case const AsyncModifier.syncStar():
+        addStatement(new Return(new Call(endOfIteration, [])));
+        break;
+      case const AsyncModifier.asyncStar():
+        addStatement(js.statement(
+            "return #streamHelper(null, null, $controllerName, null)",
+            {"streamHelper": streamHelper}));
+        break;
+      default:
+        diagnosticListener.internalError(spannable,
+            "Internal error, unexpected asyncmodifier $async");
+    }
+  }
+
+  /// The initial call to [thenHelper]/[streamHelper].
+  ///
+  /// There is no value to await/yield, so the first argument is `null` and
+  /// also the errorCallback is `null`.
+  ///
+  /// Returns the [Future]/[Stream] coming from [completerName]/
+  /// [controllerName].
+  Statement generateInitializer() {
+    if (isAsync) {
+      return js.statement(
+          "return #thenHelper(null, $helperName, $completerName, null);",
+                   {"thenHelper": thenHelper});
+    } else if (isAsyncStar) {
+      return js.statement(
+          "return #streamHelper(null, $helperName, $controllerName, null);",
+                   {"streamHelper": streamHelper});
+    } else {
+      throw diagnosticListener.internalError(spannable,
+          "Unexpected asyncModifier: $async");
+    }
+  }
+
+  /// Rewrites an async/sync*/async* function to a normal Javascript function.
+  ///
+  /// The control flow is flattened by simulating 'goto' using a switch in a
+  /// loop and a state variable [gotoName] inside a helper-function that can be
+  /// called back by [thenHelper]/[streamHelper]/the [Iterator].
+  ///
+  /// Local variables are hoisted outside the helper.
+  ///
+  /// Awaits and yields in async/async* are simulated by calling

[floitsch, 2015/02/05 20:17:21]

are translated to a call to ...

Similarly remove "simulate" in the rest.

[floitsch, 2015/02/05 20:17:21]

Maybe:
Awaits in async/async* are translated to code that remembers the current
location (so the function can resume from where it was) followed by a call to
the [thenHelper]. The helper sets up the waiting for the awaited value and
returns a future which is immediately returned by the translated await.

Yields in async* are translated to a call to the [streamHelper]. They, too, need
to be prepared to be interrupted in case the stream is paused.

[sigurdm, 2015/02/06 14:26:33]

Done.

[sigurdm, 2015/02/06 14:26:33]

Better, thanks.

+  /// [thenHelper]/[streamHelper].
+  ///
+  /// Yield/yield* in a sync* function is simulated by returning the value.

[floitsch, 2015/02/05 20:17:21]

Yield/yield* in a sync* function is translated to a return of the value, wrapped
into a "IterationMarker" that signals the type (yield or yield*). Sync*
functions are executed on demand (when the user requests a value) by the
Iterable which knows how to handle these values.

[sigurdm, 2015/02/06 14:26:32]

Done.

+  ///
+  /// Simplified examples (not the exact translation, but intended to show the
+  /// ideas):
+  ///
+  /// function (x, y, z) async {
+  ///   var p = await foo();
+  ///   return bar(p);
+  /// }
+  ///
+  /// Becomes:
+  ///
+  /// function(x, y, z) {
+  ///   var goto = 0, returnValue, completer = new Completer(), p;
+  ///   function helper(result) {
+  ///     while (true) {
+  ///       switch (goto) {
+  ///         case 0:
+  ///           goto = 1

[floitsch, 2015/02/05 20:17:21]

Add comment, that "goto = 1" remembers the location of where to continue.

[sigurdm, 2015/02/06 14:26:33]

Done.

+  ///           return thenHelper(foo(), helper, completer, null);
+  ///         case 1:
+  ///           p = result;
+  ///           returnValue = bar(p);
+  ///           goto = 2;
+  ///           break;
+  ///         case 2:
+  ///           return thenHelper(returnValue, null, completer, null)
+  ///       }
+  ///     }
+  ///     return thenHelper(null, helper, completer, null);
+  ///   }
+  /// }
+  ///
+  /// Try/catch is implemented by maintaining [handlerName] to contain the label
+  /// of the current handler. The switch is nested inside a try/catch that will
+  /// redirect the flow to the current handler.
+  ///
+  /// Finally is implemented by letting the translation of each flow-path that

[floitsch, 2015/02/05 20:17:21]

A `finally` clause is compiled similar to normal code, with the additional
complexity that `finally` clauses need to know where to jump to after the clause
is done. In the translation, each flow-path that enters a `finally` sets up the
variable [nextName] with a stack of finallies and a final jump-target (exit,
catch, ...).

[floitsch, 2015/02/05 20:17:22]

`Finally`

[sigurdm, 2015/02/06 14:26:32]

Acknowledged.

[sigurdm, 2015/02/06 14:26:32]

Done.

+  /// will enter a finally set up the variable [nextName] with a stack of
+  /// finally blocks to exit via.
+  ///
+  /// function (x, y, z) async {
+  ///   try {
+  ///     try {
+  ///       throw "error";
+  ///     } finally {
+  ///       finalize1();
+  ///     }
+  ///   } catch (e) {
+  ///     handle(e);
+  ///   } finally {
+  ///     finalize2();
+  ///   }
+  /// }
+  ///
+  /// Translates into (besides the fact that structures not containing
+  /// await/yield/yield* are left intact):
+  ///
+  /// function(x, y, z) {
+  ///   var goto = 0;
+  ///   var returnValue;
+  ///   var completer = new Completer();
+  ///   var handler = null;
+  ///   var p;
+  ///   function helper(result) {

[floitsch, 2015/02/05 20:17:23]

comment that "result" could be either the value of the awaited future, or an
error in error-case.

[sigurdm, 2015/02/06 14:26:34]

Done.

+  ///     while (true) {
+  ///       try {
+  ///         switch (goto) {
+  ///           case 0:
+  ///             handler = 4; // The outer catch-handler
+  ///             handler = 1; // The inner (implicit) catch-handler
+  ///             throw "error";
+  ///             next = [3];

[floitsch, 2015/02/05 20:17:20]

// After the finally (2) continue normally after the try.

[sigurdm, 2015/02/06 14:26:33]

Done.

+  ///             goto = 2;
+  ///             break;
+  ///           case 1: // catch handler for inner try (implicit).

[floitsch, 2015/02/05 20:17:20]

(implicit) catch handler for the inner try.

[sigurdm, 2015/02/06 14:26:33]

Done.

+  ///             next = [3]; // destination after the finally.

[floitsch, 2015/02/05 20:17:22]

why is this not "next = [4]" ?
If we enter this exception handler, we are encountering an exception, so we have
to eventually reach the outer exception handler (which seems to be "4").

[sigurdm, 2015/02/06 14:26:33]

Good point, this is a bug indeed.

+  ///             // fall-though to the finally handler.
+  ///           case 2: // finally for inner try
+  ///             handler = 4; // catch-handler for outer try.
+  ///             finalize1();
+  ///             goto = next.pop();
+  ///             break;
+  ///           case 3: // exiting inner try.
+  ///             next = [6];
+  ///             goto = 5; // finally handler for outer try.

[floitsch, 2015/02/05 20:17:22]

Why is there no break?

[sigurdm, 2015/02/06 14:26:32]

It is a mistake.

+  ///           case 4: // catch handler for outer try.
+  ///             e = result;
+  ///             handle(e);
+  ///             goto = 4;

[floitsch, 2015/02/05 20:17:21]

why is this here?
It looks unnecessary anyway. Even if we happen to generate it, keep it out
(unless there is a meaning to it).

// Fall through to finally.

[sigurdm, 2015/02/06 14:26:32]

Another mistake

+  ///           case 5: // finally handler for outer try.
+  ///             handler = null;
+  ///             finalize2();
+  ///             goto = next.pop();
+  ///             break;
+  ///           case 6: // Exiting outer try.
+  ///           case 7: // return
+  ///             return thenHelper(returnValue, null, completer, null);
+  ///         }
+  ///       } catch (error) {
+  ///         result = error;
+  ///         goto = handler;
+  ///       }
+  ///     }
+  ///     return thenHelper(null, helper, completer, null);
+  ///   }
+  /// }
+  ///
+  @override
+  Expression visitFun(Fun node) {
+    if (isSync) return node;
+
+    beginLabel(newLabel("Function start"));
+    // AsyncStar needs a returnlabel for its handling of cancelation. See
+    // [visitDartYield].
+    returnLabel = analysis.hasExplicitReturns || isAsyncStar
+        ? newLabel("Return")
+        : null;
+    Statement body = node.body;
+    targetsAndTries.add(node);
+    visitStatement(body);
+    targetsAndTries.removeLast();
+    addReturn();
+
+    List<SwitchClause> clauses = labelledParts.keys.map((label) {
+      return new Case(
+          number(label),
+          new Block(labelledParts[label]));
+    }).toList();
+    Statement helperBody = new Switch(new VariableUse(gotoName), clauses);
+    if (hasJumpThoughOuterLabel) {
+      helperBody = js.statement("$outerLabelName: #", [helperBody]);
+    }
+    if (hasTryBlocks) {
+      helperBody = js.statement("""
+          try {
+            #body
+          } catch ($errorName){
+            if ($handlerName === null)
+              throw $errorName;
+            $resultName = $errorName;
+            $gotoName = $handlerName;
+          }""", {"body": helperBody});
+    }
+    List<VariableInitialization> inits = <VariableInitialization>[];
+
+    VariableInitialization makeInit(String name, Expression initValue) {
+      return new VariableInitialization(new VariableDeclaration(name),
+                                        initValue);
+    }
+
+    inits.add(makeInit(gotoName, number(0)));
+    if (isAsync) {
+      inits.add(makeInit(completerName, new New(newCompleter, [])));
+    } else if (isAsyncStar) {
+      inits.add(makeInit(controllerName,
+                         new Call(newController,
+                                  [new VariableUse(helperName)])));
+    }
+    if (hasTryBlocks) {
+      inits.add(makeInit(handlerName, new LiteralNull()));
+    }
+    if (hasJumpThroughFinally) {
+      inits.add(makeInit(nextName, null));
+    }
+    if (analysis.hasExplicitReturns && isAsync) {
+      inits.add(makeInit(returnValueName, null));
+    }
+    if (analysis.hasThis) {
+      inits.add(makeInit(selfName, new This()));
+    }
+    inits.addAll(localVariables.map(
+        (VariableDeclaration decl) => new VariableInitialization(decl, null)));
+    inits.addAll(new Iterable.generate(tempVarHighWaterMark,
+        (int i) => makeInit(useTempVar(i + 1).name, null)));
+    VariableDeclarationList varDecl = new VariableDeclarationList(inits);
+    if (isSyncStar) {
+      return js("""
+          function (#params) {
+            return new #newIterable(function () {
+              #varDecl;
+              return function $helperName($resultName) {
+                while (true)
+                  #helperBody;
+              };
+            });
+          }
+          """, {"params": node.params,
+                "helperBody": helperBody,
+                "varDecl": varDecl,
+                "newIterable": newIterable});
+    }
+    return js("""
+        function (#params) {
+          #varDecl;
+          function $helperName($resultName) {
+            while (true)
+              #helperBody;
+          }
+          #init;
+        }""", {"params": node.params,
+               "helperBody": helperBody,
+               "varDecl": varDecl,
+               "init": generateInitializer()});
+  }
+
+  @override
+  Expression visitAccess(PropertyAccess node) {
+    return withExpression2(
+        node.receiver,
+        node.selector,
+        (receiver, selector) => new PropertyAccess(receiver, selector));
+  }
+
+  @override
+  Expression visitArrayHole(ArrayHole node) {
+    return node;
+  }
+
+  @override
+  Expression visitArrayInitializer(ArrayInitializer node) {
+    return withExpressions(node.elements, (elements) {
+      return new ArrayInitializer(elements);
+    });
+  }
+
+  @override
+  Expression visitAssignment(Assignment node) {
+    if (!shouldTransform(node)) {
+      return new Assignment.compound(visitExpression(node.leftHandSide),
+                                     node.op,
+                                     visitExpression(node.value));
+    }
+    Expression leftHandSide = node.leftHandSide;
+    if (leftHandSide is VariableUse) {
+      return withExpression(node.value, (Expression value) {
+        return new Assignment(leftHandSide, value);
+      }, store: false);
+    } else if (leftHandSide is PropertyAccess) {
+      return withExpressions(
+          [leftHandSide.receiver, leftHandSide.selector, node.value],
+          (evaluated) {
+        return new Assignment.compound(
+            new PropertyAccess(evaluated[0], evaluated[1]),
+            node.op,
+            evaluated[2]);
+      });
+    } else {
+      throw "Unexpected assignment left hand side $leftHandSide";
+    }
+  }
+
+  /// An await is translated to a call to [thenHelper]/[streamHelper].
+  ///
+  /// See the comments of [visitFun] for an example.
+  @override
+  Expression visitAwait(Await node) {
+    assert(isAsync || isAsyncStar);
+    int afterAwait = newLabel("returning from await.");
+    withExpression(node.expression, (Expression value) {
+      addStatement(setGotoVariable(afterAwait));
+      Expression errorCallback = errorHandlerLabels.isEmpty
+          ? new LiteralNull()
+          : js("""
+            function($errorName) {
+              $gotoName = #currentHandler;
+              $helperName($errorName);
+            }""", {"currentHandler": currentErrorHandler});
+
+      addStatement(js.statement("""
+          return #thenHelper(#value,
+                             $helperName,
+                             ${isAsync ? completerName : controllerName},
+                             #errorCallback);
+          """, {"thenHelper": isAsync ? thenHelper : streamHelper,
+                "value": value,
+                "errorCallback": errorCallback}));
+    }, store: false);
+    beginLabel(afterAwait);
+    return new VariableUse(resultName);
+  }
+
+  /// Checks if [node] is the variable named [resultName].
+  ///
+  /// [resultName] is used to hold the result of a transformed computation
+  /// for example the result of awaiting, or the result of a conditional or
+  /// short-circuiting expression.
+  /// If the subexpression of some transformed node already is transformed and
+  /// visiting it returns [resultName], it is not redundantly assigned to itself
+  /// again.
+  bool isResult(Expression node) {
+    return node is VariableUse && node.name == resultName;
+  }
+
+  @override
+  Expression visitBinary(Binary node) {
+    if (shouldTransform(node.right) &&
+        (node.op == "||" || node.op == "&&")) {
+      int thenLabel = newLabel("then");
+      int joinLabel = newLabel("join");
+      withExpression(node.left, (Expression left) {
+        Statement assignLeft = isResult(left)
+            ? new Block.empty()
+            : new ExpressionStatement(
+                new Assignment(new VariableUse(resultName), left));
+        if (node.op == "||") {
+          addStatement(new If(left, gotoAndBreak(thenLabel), assignLeft));
+        } else {
+          assert(node.op == "&&");
+          addStatement(new If(left, assignLeft, gotoAndBreak(thenLabel)));
+        }
+      }, store: true);
+      addGoto(joinLabel);
+      beginLabel(thenLabel);
+      withExpression(node.right, (Expression value) {
+        if (!isResult(value)) {
+          addExpressionStatement(
+              new Assignment(new VariableUse(resultName), value));
+        }
+      }, store: false);
+      beginLabel(joinLabel);
+      return new VariableUse(resultName);
+    }
+
+    return withExpression2(
+        node.left,
+        node.right,
+        (left, right) => new Binary(node.op, left, right));
+  }
+
+  @override
+  Expression visitBlob(Blob node) {
+    return node;
+  }
+
+  @override
+  void visitBlock(Block node) {
+    for (Statement statement in node.statements) {
+      visitStatement(statement);
+    }
+  }
+
+  @override
+  void visitBreak(Break node) {
+    Node target = analysis.targets[node];
+    if (!shouldTransform(target)) {
+      addStatement(node);
+      return;
+    }
+    translateJump(target, breakLabels[target]);
+  }
+
+  @override
+  Expression visitCall(Call node) {
+    bool storeTarget = node.arguments.any(shouldTransform);
+    return withExpression(node.target, (target) {
+      return withExpressions(node.arguments, (List<Expression> arguments) {
+        return new Call(target, arguments);
+      });
+    }, store: storeTarget);
+  }
+
+  @override
+  void visitCase(Case node) {
+    return unreachable(node);
+  }
+
+  @override
+  void visitCatch(Catch node) {
+    return unreachable(node);
+  }
+
+  @override
+  void visitComment(Comment node) {
+    addStatement(node);
+  }
+
+  @override
+  Expression visitConditional(Conditional node) {
+    if (!shouldTransform(node.then) && !shouldTransform(node.otherwise)) {
+      return withExpression(node.condition, (Expression condition) {
+        return new Conditional(condition, node.then, node.otherwise);
+      });
+    }
+    int thenLabel = newLabel("then");
+    int joinLabel = newLabel("join");
+    int elseLabel = newLabel("else");
+    withExpression(node.condition, (Expression condition) {
+      addExpressionStatement(
+          new Assignment(
+              new VariableUse(gotoName),
+              new Conditional(
+                  condition,
+                  number(thenLabel),
+                  number(elseLabel))));
+    }, store: false);
+    addBreak();
+    beginLabel(thenLabel);
+    withExpression(node.then, (Expression value) {
+      if (!isResult(value)) {
+        addExpressionStatement(
+            new Assignment(new VariableUse(resultName), value));
+      }
+    }, store: false);
+    addGoto(joinLabel);
+    beginLabel(elseLabel);
+    withExpression(node.otherwise, (Expression value) {
+      if (!isResult(value)) {
+        addExpressionStatement(
+            new Assignment(new VariableUse(resultName), value));
+      }
+    }, store: false);
+    beginLabel(joinLabel);
+    return new VariableUse(resultName);
+  }
+
+  @override
+  void visitContinue(Continue node) {
+    Node target = analysis.targets[node];
+    if (!shouldTransform(target)) {
+      addStatement(node);
+      return;
+    }
+    translateJump(target, continueLabels[target]);
+  }
+
+  /// Emits a break statement that exits the big switch statement.
+  void addBreak() {
+    if (insideUntranslatedBreakable) {
+      hasJumpThoughOuterLabel = true;
+      addStatement(new Break(outerLabelName));
+    } else {
+      addStatement(new Break(null));
+    }
+  }
+
+  /// Common code for handling break, continue, return.
+  ///
+  /// It is necessary to run all nesting finally-handlers between the jump and
+  /// the target. For that [nextName] is used as a stack of places to go.

[floitsch, 2015/02/05 20:17:21]

Maybe reference visitFun.

[sigurdm, 2015/02/06 14:26:32]

Done.

+  void translateJump(Node target, int targetLabel) {
+    // Compute a stack of all the 'finally' nodes that must be visited before
+    // the jump.
+    // The bottom of the stack is the label where the jump goes to.
+    List<int> jumpStack = new List<int>();
+    for (Node node in targetsAndTries.reversed) {
+      if (node is Try) {
+        assert(node.finallyPart != null);
+        jumpStack.add(finallyLabels[node]);
+      } else if (node == target) {
+        jumpStack.add(targetLabel);
+        break;
+      }
+      // Ignore other nodes.
+    }
+    jumpStack = jumpStack.reversed.toList();
+    // As the program jumps directly to the top of the stack, it is taken off
+    // now.
+    int firstTarget = jumpStack.removeLast();
+    if (jumpStack.isNotEmpty) {
+      hasJumpThroughFinally = true;
+      Expression jsJumpStack = new ArrayInitializer(
+          jumpStack.map((int label) => number(label))
+                      .toList());
+      addStatement(js.statement("$nextName = #", [jsJumpStack]));
+    }
+    addGoto(firstTarget);
+  }
+
+  @override
+  void visitDefault(Default node) => unreachable(node);
+
+  @override
+  void visitDo(Do node) {
+    if (!shouldTransform(node)) {
+      bool oldInsideUntranslatedBreakable = insideUntranslatedBreakable;
+      insideUntranslatedBreakable = true;
+      withExpression(node.condition, (Expression condition) {
+        addStatement(new Do(translateInBlock(node.body), condition));
+      }, store: false);
+      insideUntranslatedBreakable = oldInsideUntranslatedBreakable;
+      return;
+    }
+    int startLabel = newLabel("do body");
+
+    int continueLabel = newLabel("do condition");
+    continueLabels[node] = continueLabel;
+
+    int afterLabel = newLabel("after do");
+    breakLabels[node] = afterLabel;
+
+    beginLabel(startLabel);
+
+    targetsAndTries.add(node);
+    visitStatement(node.body);
+    targetsAndTries.removeLast();
+
+    beginLabel(continueLabel);
+    withExpression(node.condition, (Expression condition) {
+      addStatement(new If.noElse(condition,
+                                 gotoAndBreak(startLabel)));
+    }, store: false);
+    beginLabel(afterLabel);
+  }
+
+  @override
+  void visitEmptyStatement(EmptyStatement node) {
+    addStatement(node);
+  }
+
+  void visitExpressionInStatementContext(Expression node) {
+    if (node is VariableDeclarationList) {
+      // Treat VariableDeclarationList as a statement.
+      visitVariableDeclarationList(node);
+    } else {
+      visitExpressionIgnoreResult(node);
+    }
+  }
+
+  @override
+  void visitExpressionStatement(ExpressionStatement node) {
+    visitExpressionInStatementContext(node.expression);
+  }
+
+  @override
+  void visitFor(For node) {
+    if (!shouldTransform(node) ) {
+      bool oldInsideUntranslated = insideUntranslatedBreakable;
+      insideUntranslatedBreakable = true;
+      // Note that node.init, node.condition, node.update all can be null, but
+      // withExpressions handles that.
+      withExpressions([node.init, node.condition, node.update],
+          (List<Expression> transformed) {
+        addStatement(new For(transformed[0],
+                             transformed[1],
+                             transformed[2],
+                             translateInBlock(node.body)));
+      });
+      insideUntranslatedBreakable = oldInsideUntranslated;
+      return;
+    }
+
+    if (node.init != null) {
+      visitExpressionInStatementContext(node.init);
+    }
+    int startLabel = newLabel("for condition");
+    // If there is no update, continuing the loop is the same as going to the
+    // start.
+    int continueLabel = (node.update == null)
+        ? startLabel
+        : newLabel("for update");
+    continueLabels[node] = continueLabel;
+    int afterLabel = newLabel("after for");
+    breakLabels[node] = afterLabel;
+    beginLabel(startLabel);
+    Expression condition = node.condition;
+    if (condition == null ||
+        (condition is LiteralBool && condition.value == true)) {
+      // No condition is the same as true. So the check is not needed.

[floitsch, 2015/02/05 20:17:20]

I'm not sure I understand the comment.
Remove?

[sigurdm, 2015/02/06 14:26:33]

Done.

+      addStatement(new Comment("trivial condition"));
+    } else {
+      withExpression(condition, (Expression condition) {
+        addStatement(new If.noElse(new Prefix("!", condition),
+                                   gotoAndBreak(afterLabel)));
+      }, store: false);
+    }
+    targetsAndTries.add(node);
+    visitStatement(node.body);
+    targetsAndTries.removeLast();
+    if (node.update != null) {
+      beginLabel(continueLabel);
+      visitExpressionIgnoreResult(node.update);
+    }
+    addGoto(startLabel);
+    beginLabel(afterLabel);
+  }
+
+  @override
+  void visitForIn(ForIn node) {
+    // The dart output currently never uses for-in loops.
+    throw "Javascript for-in not implemented yet in the await transformation";
+  }
+
+  @override
+  void visitFunctionDeclaration(FunctionDeclaration node) {
+    unsupported(node);
+  }
+
+  // Only used for code where `!shouldTransform(node)`.
+  Block translateInBlock(Statement node) {
+    assert(!shouldTransform(node));
+    List<Statement> oldBuffer = currentStatementBuffer;
+    currentStatementBuffer = new List();
+    List<Statement> resultBuffer = currentStatementBuffer;
+    visitStatement(node);
+    currentStatementBuffer = oldBuffer;
+    return new Block(resultBuffer);
+  }
+
+  @override
+  void visitIf(If node) {
+    if (!shouldTransform(node.then) && !shouldTransform(node.otherwise)) {
+      withExpression(node.condition, (Expression condition) {
+        addStatement(
+            new If(
+                condition,
+                translateInBlock(node.then),
+                translateInBlock(node.otherwise)));
+      }, store: false);
+      return;
+    }
+    int thenLabel = newLabel("then");
+    int joinLabel = newLabel("join");
+    int elseLabel =
+        node.otherwise is EmptyStatement ? joinLabel : newLabel("else");
+
+    withExpression(node.condition, (Expression condition) {
+      addExpressionStatement(
+          new Assignment(
+              new VariableUse(gotoName),
+              new Conditional(
+                  condition,
+                  number(thenLabel),
+                  number(elseLabel))));
+    }, store: false);
+    addBreak();
+    beginLabel(thenLabel);
+    visitStatement(node.then);
+    if (node.otherwise is! EmptyStatement) {
+      addGoto(joinLabel);
+      beginLabel(elseLabel);
+      visitStatement(node.otherwise);
+    }
+    beginLabel(joinLabel);
+  }
+
+  @override
+  visitInterpolatedExpression(InterpolatedExpression node) => unsupported(node);
+
+  @override
+  visitInterpolatedLiteral(InterpolatedLiteral node) => unsupported(node);
+
+  @override
+  visitInterpolatedParameter(InterpolatedParameter node) => unsupported(node);
+
+  @override
+  visitInterpolatedSelector(InterpolatedSelector node) => unsupported(node);
+
+  @override
+  visitInterpolatedStatement(InterpolatedStatement node) => unsupported(node);
+
+  @override
+  void visitLabeledStatement(LabeledStatement node) {
+    if (!shouldTransform(node)) {
+      addStatement(
+          new LabeledStatement(node.label, translateInBlock(node.body)));
+      return;
+    }
+    int breakLabel = newLabel("break ${node.label}");
+    int continueLabel = newLabel("continue ${node.label}");
+    breakLabels[node] = breakLabel;
+    continueLabels[node] = continueLabel;
+
+    beginLabel(continueLabel);
+    targetsAndTries.add(node);
+    visitStatement(node.body);
+    targetsAndTries.removeLast();
+    beginLabel(breakLabel);
+  }
+
+  @override
+  Expression visitLiteralBool(LiteralBool node) => node;
+
+  @override
+  visitLiteralExpression(LiteralExpression node) => unsupported(node);
+
+  @override
+  Expression visitLiteralNull(LiteralNull node) => node;
+
+  @override
+  Expression visitLiteralNumber(LiteralNumber node) => node;
+
+  @override
+  visitLiteralStatement(LiteralStatement node) => unsupported(node);
+
+  @override
+  Expression visitLiteralString(LiteralString node) => node;
+
+  @override
+  visitNamedFunction(NamedFunction node) {
+    unsupported(node);
+  }
+
+  @override
+  Expression visitNew(New node) {
+    bool storeTarget = node.arguments.any(shouldTransform);
+    return withExpression(node.target, (target) {
+      return withExpressions(node.arguments, (List<Expression> arguments) {
+        return new New(target, arguments);
+      });
+    }, store: storeTarget);
+  }
+
+  @override
+  Expression visitObjectInitializer(ObjectInitializer node) {
+    return withExpressions(
+        node.properties.map((Property property) => property.value).toList(),
+        (List<Node> values) {
+      List<Property> properties = new List.generate(values.length, (int i) {
+        return new Property(node.properties[i].name, values[i]);
+      });
+      return new ObjectInitializer(properties);
+    });
+  }
+
+  @override
+  visitParameter(Parameter node) => unreachable(node);
+
+  @override
+  Expression visitPostfix(Postfix node) {
+    if (node.op == "++" || node.op == "--") {
+      Expression argument = node.argument;
+      if (argument is VariableUse) {
+        return new Postfix(node.op, argument);
+      } else if (argument is PropertyAccess) {
+        return withExpression2(argument.receiver, argument.selector,
+            (receiver, selector) {
+          return new Postfix(node.op, new PropertyAccess(receiver, selector));
+        });
+      } else {
+        throw "Unexpected postfix ${node.op} "
+              "operator argument ${node.argument}";
+      }
+    }
+    return withExpression(
+        node.argument,
+        (Expression argument) => new Postfix(node.op, argument),
+        store: false);
+  }
+
+  @override
+  Expression visitPrefix(Prefix node) {
+    if (node.op == "++" || node.op == "--") {
+      Expression argument = node.argument;
+      if (argument is VariableUse) {
+        return new Prefix(node.op, argument);
+      } else if (argument is PropertyAccess) {
+        return withExpression2(argument.receiver, argument.selector,
+            (receiver, selector) {
+          return new Prefix(node.op, new PropertyAccess(receiver, selector));
+        });
+      } else {
+        throw "Unexpected prefix ${node.op} operator "
+              "argument ${node.argument}";
+      }
+    }
+    return withExpression(
+        node.argument,
+        (Expression argument) => new Prefix(node.op, argument),
+        store: false);
+  }
+
+  @override
+  visitProgram(Program node) => unsupported(node);
+
+  @override
+  Property visitProperty(Property node) {
+    return withExpression(
+        node.value,
+        (Expression value) => new Property(node.name, value),
+        store: false);
+  }
+
+  @override
+  Expression visitRegExpLiteral(RegExpLiteral node) => node;
+
+  @override
+  void visitReturn(Return node) {
+    assert(node.value == null || !isSyncStar && !isAsyncStar);
+    Node target = analysis.targets[node];
+    if (node.value != null) {
+      withExpression(node.value, (Expression value) {
+        addStatement(js.statement("$returnValueName = #", [value]));
+      }, store: false);
+    }
+    translateJump(target, returnLabel);
+  }
+
+  @override
+  void visitSwitch(Switch node) {
+    if (!node.cases.any(shouldTransform)) {
+      // If only the key has an await, translation can be simplified.
+      bool oldInsideUntranslated = insideUntranslatedBreakable;
+      insideUntranslatedBreakable = true;
+      withExpression(node.key, (Expression key) {
+        List<SwitchClause> cases = node.cases.map((SwitchClause clause) {
+          if (clause is Case) {
+            return new Case(clause.expression, translateInBlock(clause.body));
+          } else if (clause is Default) {
+            return new Default(translateInBlock(clause.body));
+          }
+        }).toList();
+        addStatement(new Switch(key, cases));
+      }, store: false);
+      insideUntranslatedBreakable = oldInsideUntranslated;
+      return;
+    }
+    int before = newLabel("switch");
+    int after = newLabel("after switch");
+    breakLabels[node] = after;
+
+    beginLabel(before);
+    List<int> labels = new List<int>(node.cases.length);
+
+    if (!node.cases.every(
+        (SwitchClause x) => !(x is Case && shouldTransform(x.expression)))) {
+      int defaultIndex = null; // Null means no default was found.
+      // If there is an await in one of the keys, a chain of ifs has to be used.
+
+      withExpression(node.key, (Expression key) {
+        int i = 0;
+        for (SwitchClause clause in node.cases) {
+          if (clause is Default) {
+            // The goto for the default case is added after all non-default
+            // clauses have been handled.
+            defaultIndex = i;
+            labels[i] = newLabel("default");
+            continue;
+          } else if (clause is Case) {
+            labels[i] = newLabel("case");
+            withExpression(clause.expression, (expression) {
+              addStatement(
+                  new If.noElse(new Binary("===", key, expression),
+                                gotoAndBreak(labels[i])));
+            }, store: false);
+          }
+          i++;
+        }
+      }, store: true);
+
+      if (defaultIndex == null) {
+        addGoto(after);
+      } else {
+        addGoto(labels[defaultIndex]);
+      }
+
+    } else {
+      bool hasDefault = false;
+      int i = 0;
+      List<SwitchClause> clauses = new List<SwitchClause>();
+      for (SwitchClause clause in node.cases) {
+        if (clause is Case) {
+          labels[i] = newLabel("case");
+          clauses.add(new Case(clause.expression, gotoAndBreak(labels[i])));
+        } else if (i is Default) {
+          labels[i] = newLabel("default");
+          clauses.add(new Default(gotoAndBreak(labels[i])));
+          hasDefault = true;
+        } else {
+          diagnosticListener.internalError(spannable,
+                                           "Unknown clause type $clause");
+        }
+        i++;
+      }
+      withExpression(node.key, (Expression key) {
+        addStatement(new Switch(key, clauses));
+      }, store: false);
+      if (!hasDefault) {
+        addGoto(after);
+      }
+    }
+
+    targetsAndTries.add(node);
+    for (int i = 0; i < labels.length; i++) {
+      beginLabel(labels[i]);
+      visitStatement(node.cases[i].body);
+    }
+    beginLabel(after);
+    targetsAndTries.removeLast();
+  }
+
+  @override
+  Expression visitThis(This node) {
+    return new VariableUse(selfName);
+  }
+
+  @override
+  void visitThrow(Throw node) {
+    withExpression(node.expression, (Expression expression) {
+      addStatement(new Throw(expression));
+    }, store: false);
+  }
+
+  setHandler() {
+    addExpressionStatement(
+        new Assignment(new VariableUse(handlerName), currentErrorHandler));
+  }
+
+  @override
+  void visitTry(Try node) {

[floitsch, 2015/02/05 20:17:21]

Reference visitFun.

[sigurdm, 2015/02/06 14:26:32]

Done.

+    if (!shouldTransform(node)) {
+      Block body = translateInBlock(node.body);
+      Catch catchPart = (node.catchPart == null)
+          ? null
+          : new Catch(node.catchPart.declaration,
+                    translateInBlock(node.catchPart.body));
+      Block finallyPart = (node.finallyPart == null)
+          ? null
+          : translateInBlock(node.finallyPart);
+      addStatement(new Try(body, catchPart, finallyPart));
+      return;
+    }
+    hasTryBlocks = true;
+    int handlerLabel = newLabel("catch");
+    int finallyLabel = newLabel("finally");
+    int afterFinallyLabel = newLabel("after finally");
+    errorHandlerLabels.add(handlerLabel);
+    // Set the error handler here. It must be cleared on every path out;
+    // normal and error exit.
+    setHandler();

[floitsch, 2015/02/05 20:17:22]

maybe rename to setErrorHandler

[sigurdm, 2015/02/06 14:26:33]

Done.

+    if (node.finallyPart != null) {
+      finallyLabels[node] = finallyLabel;
+      targetsAndTries.add(node);
+    }
+    visitStatement(node.body);
+    errorHandlerLabels.removeLast();
+    addStatement(js.statement("$nextName = [#];",
+        [number(afterFinallyLabel)]));
+    if (node.finallyPart != null) {
+      // The handler is set as the first thing in the finally block.
+      addGoto(finallyLabel);
+    } else {
+      setHandler();
+      addGoto(afterFinallyLabel);
+    }
+    beginLabel(handlerLabel);
+    if (node.catchPart != null) {
+      setHandler();
+      // The catch declaration name can shadow outer variables, so a fresh name
+      // is needed to avoid collisions.

[floitsch, 2015/02/05 20:17:22]

collisions. See Ecma 262, 3rd edition, section 12.14.

[sigurdm, 2015/02/06 14:26:33]

Done.

+      String errorRename = freshName(node.catchPart.declaration.name);
+      localVariables.add(new VariableDeclaration(errorRename));
+      variableRenamings.add(
+          new Pair(node.catchPart.declaration.name, errorRename));
+      addExpressionStatement(
+          new Assignment(new VariableUse(errorRename),
+                         new VariableUse(resultName)));
+      visitStatement(node.catchPart.body);
+      variableRenamings.removeLast();
+    }
+    if (node.finallyPart != null) {
+      setHandler();
+      targetsAndTries.removeLast();
+      // This belongs to the catch-part, but is only needed if there is a
+      // finally therefore it is in this branch.
+      // This is needed it even if there is no explicit catch-branch, because
+      // if an exception is raised the finally part has to be run.

[floitsch, 2015/02/05 20:17:21]

I don't get this.

If the try-body executes without a throw it sets the nextName in 1656 (to
afterFinallyLabel).
So for non-erroneous execution the nextName is already set up.
That means that we only need to set it to 'after-finally' for the catch, so we
should be able to just do it as last instruction in the 'catch' section.

What is missing, though: how does a catch-clause reach it's finally if it throws
itself.
I believe it has to set up a $nextName to point to the current finally followed
by the next catch-handler (possibly passing through other finallies).

Basically:

<try>:
  handler = <handlerId>;
  ...
  nextTargets = [<afterFinally>]
<catch>:
  handler = <finally>;  // Go to finally if we throw.
  nextTargets = [<otherFinally>, <outerCatch>];  // And then to the next catch.
  <if explicit catch clause>
    catchHandler();
    nextTargets = [<afterFinally>]
  <endif>
<finally>:
  handler = <outerHandler>;

[sigurdm, 2015/02/06 14:26:34]

I agree - this is a bug. Thanks for spotting.

+      addStatement(js.statement("$nextName = [#];",
+          [number(afterFinallyLabel)]));
+      beginLabel(finallyLabel);
+      setHandler();
+      visitStatement(node.finallyPart);
+      addStatement(new Comment("// goto the next finally handler"));
+      addStatement(js.statement("$gotoName = $nextName.pop();"));
+      addBreak();
+    }
+    beginLabel(afterFinallyLabel);
+  }
+
+  @override
+  visitVariableDeclaration(VariableDeclaration node) {
+    unreachable(node);
+  }
+
+  @override
+  void visitVariableDeclarationList(VariableDeclarationList node) {
+    // Declaration of local variables is hoisted outside the helper but the
+    // initialization is done here.
+    for (VariableInitialization initialization in node.declarations) {
+      VariableDeclaration declaration = initialization.declaration;
+      localVariables.add(declaration);
+      if (initialization.value != null) {
+        withExpression(initialization.value, (Expression value) {
+          addStatement(
+              new ExpressionStatement(new Assignment(declaration, value)));
+        }, store: false);
+      }
+    }
+  }
+
+  @override
+  void visitVariableInitialization(VariableInitialization node) {
+    unreachable(node);
+  }
+
+  @override
+  Expression visitVariableUse(VariableUse node) {
+    Pair<String, String> renaming = variableRenamings.lastWhere(
+        (Pair renaming) => renaming.a == node.name,
+        orElse: () => null);
+    if (renaming == null) return node;
+    return new VariableUse(renaming.b);
+  }
+
+  @override
+  void visitWhile(While node) {
+    if (!shouldTransform(node)) {
+      bool oldInsideUntranslated = insideUntranslatedBreakable;
+      insideUntranslatedBreakable = true;
+      withExpression(node.condition, (Expression condition) {
+        addStatement(new While(condition, translateInBlock(node.body)));
+      }, store: false);
+      insideUntranslatedBreakable = oldInsideUntranslated;
+      return;
+    }
+    int continueLabel = newLabel("while condition");
+    continueLabels[node] = continueLabel;
+    beginLabel(continueLabel);
+
+    int afterLabel = newLabel("after while");
+    breakLabels[node] = afterLabel;
+    Expression condition = node.condition;
+    // If the condition is `true`, a test is not needed.
+    if (!(condition is LiteralBool && condition.value == true)) {
+      withExpression(node.condition, (Expression condition) {
+        addStatement(new If.noElse(new Prefix("!", condition),
+                                   gotoAndBreak(afterLabel)));
+      }, store: false);
+    }
+    targetsAndTries.add(node);
+    visitStatement(node.body);
+    targetsAndTries.removeLast();
+    addGoto(continueLabel);
+    beginLabel(afterLabel);
+  }
+
+  /// Translates a yield/yield* in an sync* function.

[floitsch, 2015/02/05 20:17:21]

in a sync*

[sigurdm, 2015/02/06 14:26:34]

Done.

+  ///
+  /// `yield` in a sync* function just returns [value].
+  /// `yield*` wraps [value] in a [yieldStarExpression] and returns it.
+  void addSyncYield(DartYield node, Expression expression) {
+    assert(isSyncStar);
+    if (node.hasStar) {
+      addStatement(new Return(new Call(yieldStarExpression, [expression])));
+    } else {
+      addStatement(new Return(expression));
+    }
+  }
+
+  /// Translates a yield/yield* in an async* function.
+  ///
+  /// yield/yield* in an async* function is translated much like the `await` is
+  /// translated in [visitAwait], only the object is wrapped in a
+  /// [yieldExpression]/[yieldStarExpression] to let [streamHelper] distinguish.
+  ///
+  /// Because there is no Future that can fail (as there is in await) null is
+  /// passed as the errorCallback.
+  void addAsyncYield(DartYield node, Expression expression) {
+    assert(isAsyncStar);
+    // Find all the finally blocks that should be performed if the stream is
+    // canceled during the yield.
+    // At the bottom of the stack is the return label.
+    List<int> enclosingFinallyLabels = <int>[returnLabel];

[floitsch, 2015/02/05 20:17:22]

I think "exitLabel" would read easier. (Here in the other places).

[sigurdm, 2015/02/06 14:26:33]

Done.

+    enclosingFinallyLabels.addAll(targetsAndTries
+        .where((Node node) => node is Try)
+        .map((Try node) => finallyLabels[node]));
+    int destinationOnCancel = enclosingFinallyLabels.removeLast();
+    ArrayInitializer finallyListInitializer =
+        new ArrayInitializer(enclosingFinallyLabels
+            .map((int label) => number(label))

[floitsch, 2015/02/05 20:17:22]

.map(number).toList()

and yes: we should have a prefix for the js-library.

[sigurdm, 2015/02/06 14:26:32]

Done.

+            .toList());
+    addStatement(js.statement("""
+        return #streamHelper(#yieldExpression(#expression),
+            $helperName, $controllerName, function () {
+              if (#notEmptyFinallyList)
+                $nextName = #finallyList;
+              $gotoName = #destinationOnCancel;
+              $helperName();
+            });""",
+        {"streamHelper": streamHelper,
+         "yieldExpression": node.hasStar
+              ? yieldStarExpression
+              : yieldExpression,
+         "expression": expression,
+         "notEmptyFinallyList": enclosingFinallyLabels.isNotEmpty,
+         "finallyList": finallyListInitializer,
+         "destinationOnCancel": number(destinationOnCancel)}));
+  }
+
+  @override
+  void visitDartYield(DartYield node) {
+    assert(isSyncStar || isAsyncStar);
+    int label = newLabel("after yield");
+    // Don't do a break here for the goto, but instead a return in either
+    // addSynYield or addAsyncYield.
+    withExpression(node.expression, (Expression expression) {
+      addStatement(setGotoVariable(label));
+      if (isSyncStar) {
+        addSyncYield(node, expression);
+      } else {
+        addAsyncYield(node, expression);
+      }
+    }, store: false);
+    beginLabel(label);
+  }
+}
+
+/// Finds out
+///
+/// - which expressions have yield or await nested in them.
+/// - targets of jumps
+/// - a set of used names.
+/// - if any [This]-expressions are used.
+class PreTranslationAnalysis extends NodeVisitor<bool> {
+  Set<Node> hasAwaitOrYield = new Set<Node>();
+
+  Map<Node, Node> targets = new Map<Node, Node>();
+  List<Node> loopsAndSwitches = new List<Node>();
+  List<LabeledStatement> labelledStatements = new List<LabeledStatement>();
+  Set<String> usedNames = new Set<String>();
+
+  bool hasExplicitReturns = false;
+
+  bool hasThis = false;
+
+  // The function currently being analyzed.
+  Fun currentFunction;
+
+  // For error messages.
+  final Function unsupported;
+
+  PreTranslationAnalysis(void this.unsupported(Node node));
+
+  bool visit(Node node) {
+    bool containsAwait = node.accept(this);
+    if (containsAwait) {
+      hasAwaitOrYield.add(node);
+    }
+    return containsAwait;
+  }
+
+  analyze(Fun node) {
+    currentFunction = node;
+    node.params.forEach(visit);
+    visit(node.body);
+  }
+
+  @override
+  bool visitAccess(PropertyAccess node) {
+    bool receiver = visit(node.receiver);
+    bool selector = visit(node.selector);
+    return receiver || selector;
+  }
+
+  @override
+  bool visitArrayHole(ArrayHole node) {
+    return false;
+  }
+
+  @override
+  bool visitArrayInitializer(ArrayInitializer node) {
+    bool containsAwait = false;
+    for (Expression element in node.elements) {
+      if (visit(element)) containsAwait = true;
+    }
+    return containsAwait;
+  }
+
+  @override
+  bool visitAssignment(Assignment node) {
+    bool leftHandSide = visit(node.leftHandSide);
+    bool value = (node.value == null) ? false : visit(node.value);
+    return leftHandSide || value;
+  }
+
+  @override
+  bool visitAwait(Await node) {
+    visit(node.expression);
+    return true;
+  }
+
+  @override
+  bool visitBinary(Binary node) {
+    bool left = visit(node.left);
+    bool right = visit(node.right);
+    return left || right;
+  }
+
+  @override
+  bool visitBlob(Blob node) {
+    return false;
+  }
+
+  @override
+  bool visitBlock(Block node) {
+    bool containsAwait = false;
+    for (Statement statement in node.statements) {
+      if (visit(statement)) containsAwait = true;
+    }
+    return containsAwait;
+  }
+
+  @override
+  bool visitBreak(Break node) {
+    if (node.targetLabel != null) {
+      targets[node] = labelledStatements.lastWhere(
+          (LabeledStatement statement) => statement.label == node.targetLabel);
+    } else {
+      targets[node] = loopsAndSwitches.last;
+    }
+    return false;
+  }
+
+  @override
+  bool visitCall(Call node) {
+    bool containsAwait = visit(node.target);
+    for (Expression argument in node.arguments) {
+      if (visit(argument)) containsAwait = true;
+    }
+    return containsAwait;
+  }
+
+  @override
+  bool visitCase(Case node) {
+    bool expression = visit(node.expression);
+    bool body = visit(node.body);
+    return expression || body;
+  }
+
+  @override
+  bool visitCatch(Catch node) {
+    bool declaration = visit(node.declaration);
+    bool body = visit(node.body);
+    return declaration || body;
+  }
+
+  @override
+  bool visitComment(Comment node) {
+    return false;
+  }
+
+  @override
+  bool visitConditional(Conditional node) {
+    bool condition = visit(node.condition);
+    bool then = visit(node.then);
+    bool otherwise = visit(node.otherwise);
+    return condition || then || otherwise;
+  }
+
+  @override
+  bool visitContinue(Continue node) {
+    if (node.targetLabel != null) {
+      targets[node] = labelledStatements.lastWhere(
+          (LabeledStatement stm) => stm.label == node.targetLabel);
+    } else {
+      targets[node] =
+          loopsAndSwitches.lastWhere((Node node) => node is! Switch);
+    }
+    assert(() {
+      Node target = targets[node];
+      return target is Loop ||
+          (target is LabeledStatement && target.body is Loop);
+    });
+    return false;
+  }
+
+  @override
+  bool visitDefault(Default node) {
+    return visit(node.body);
+  }
+
+  @override
+  bool visitDo(Do node) {
+    loopsAndSwitches.add(node);
+    bool body = visit(node.body);
+    bool condition = visit(node.condition);
+    loopsAndSwitches.removeLast();
+    return body || condition;
+  }
+
+  @override
+  bool visitEmptyStatement(EmptyStatement node) {
+    return false;
+  }
+
+  @override
+  bool visitExpressionStatement(ExpressionStatement node) {
+    return visit(node.expression);
+  }
+
+  @override
+  bool visitFor(For node) {
+    bool init = (node.init == null) ? false : visit(node.init);
+    bool condition = (node.condition == null) ? false : visit(node.condition);
+    bool update = (node.update == null) ? false : visit(node.update);
+    loopsAndSwitches.add(node);
+    bool body = visit(node.body);
+    loopsAndSwitches.removeLast();
+    return init || condition || update || body;
+  }
+
+  @override
+  bool visitForIn(ForIn node) {
+    bool object = visit(node.object);
+    loopsAndSwitches.add(node);
+    bool body = visit(node.body);
+    loopsAndSwitches.removeLast();
+    return object || body;
+  }
+
+  @override
+  bool visitFun(Fun node) {
+    return false;
+  }
+
+  @override
+  bool visitFunctionDeclaration(FunctionDeclaration node) {
+    return false;
+  }
+
+  @override
+  bool visitIf(If node) {
+    bool condition = visit(node.condition);
+    bool then = visit(node.then);
+    bool otherwise = visit(node.otherwise);
+    return condition || then || otherwise;
+  }
+
+  @override
+  bool visitInterpolatedExpression(InterpolatedExpression node) {
+    return unsupported(node);
+  }
+
+  @override
+  bool visitInterpolatedLiteral(InterpolatedLiteral node) {
+    return unsupported(node);
+  }
+
+  @override
+  bool visitInterpolatedParameter(InterpolatedParameter node) {
+    return unsupported(node);
+  }
+
+  @override
+  bool visitInterpolatedSelector(InterpolatedSelector node) {
+    return unsupported(node);
+  }
+
+  @override
+  bool visitInterpolatedStatement(InterpolatedStatement node) {
+    return unsupported(node);
+  }
+
+  @override
+  bool visitLabeledStatement(LabeledStatement node) {
+    usedNames.add(node.label);
+    labelledStatements.add(node);
+    bool containsAwait = visit(node.body);
+    labelledStatements.removeLast();
+    return containsAwait;
+  }
+
+  @override
+  bool visitLiteralBool(LiteralBool node) {
+    return false;
+  }
+
+  @override
+  bool visitLiteralExpression(LiteralExpression node) {
+    return unsupported(node);
+  }
+
+  @override
+  bool visitLiteralNull(LiteralNull node) {
+    return false;
+  }
+
+  @override
+  bool visitLiteralNumber(LiteralNumber node) {
+    return false;
+  }
+
+  @override
+  bool visitLiteralStatement(LiteralStatement node) {
+    return unsupported(node);
+  }
+
+  @override
+  bool visitLiteralString(LiteralString node) {
+    return false;
+  }
+
+  @override
+  bool visitNamedFunction(NamedFunction node) {
+    return false;
+  }
+
+  @override
+  bool visitNew(New node) {
+    return visitCall(node);
+  }
+
+  @override
+  bool visitObjectInitializer(ObjectInitializer node) {
+    bool containsAwait = false;
+    for (Property property in node.properties) {
+      if (visit(property)) containsAwait = true;
+    }
+    return containsAwait;
+  }
+
+  @override
+  bool visitParameter(Parameter node) {
+    usedNames.add(node.name);
+    return false;
+  }
+
+  @override
+  bool visitPostfix(Postfix node) {
+    return visit(node.argument);
+  }
+
+  @override
+  bool visitPrefix(Prefix node) {
+    return visit(node.argument);
+  }
+
+  @override
+  bool visitProgram(Program node) {
+    throw "Unexpected";
+  }
+
+  @override
+  bool visitProperty(Property node) {
+    return visit(node.value);
+  }
+
+  @override
+  bool visitRegExpLiteral(RegExpLiteral node) {
+    return false;
+  }
+
+  @override
+  bool visitReturn(Return node) {
+    hasExplicitReturns = true;
+    targets[node] = currentFunction;
+    if (node.value == null) return false;
+    return visit(node.value);
+  }
+
+  @override
+  bool visitSwitch(Switch node) {
+    loopsAndSwitches.add(node);
+    bool result = visit(node.key);
+    for (SwitchClause clause in node.cases) {
+      if (visit(clause)) result = true;
+    }
+    loopsAndSwitches.removeLast();
+    return result;
+  }
+
+  @override
+  bool visitThis(This node) {
+    hasThis = true;
+    return false;
+  }
+
+  @override
+  bool visitThrow(Throw node) {
+    return visit(node.expression);
+  }
+
+  @override
+  bool visitTry(Try node) {
+    bool body = visit(node.body);
+    bool catchPart = (node.catchPart == null) ? false : visit(node.catchPart);
+    bool finallyPart = (node.finallyPart == null)
+        ? false
+        : visit(node.finallyPart);
+    return body || catchPart || finallyPart;
+  }
+
+  @override
+  bool visitVariableDeclaration(VariableDeclaration node) {
+    usedNames.add(node.name);
+    return false;
+  }
+
+  @override
+  bool visitVariableDeclarationList(VariableDeclarationList node) {
+    bool result = false;
+    for (VariableInitialization init in node.declarations) {
+      if (visit(init)) result = true;
+    }
+    return result;
+  }
+
+  @override
+  bool visitVariableInitialization(VariableInitialization node) {
+    return visitAssignment(node);
+  }
+
+  @override
+  bool visitVariableUse(VariableUse node) {
+    usedNames.add(node.name);
+    return false;
+  }
+
+  @override
+  bool visitWhile(While node) {
+    loopsAndSwitches.add(node);
+    bool condition = visit(node.condition);
+    bool body = visit(node.body);
+    loopsAndSwitches.removeLast();
+    return condition || body;
+  }
+
+  @override
+  bool visitDartYield(DartYield node) {
+    visit(node.expression);
+    return true;
+  }
+}