Implementation of async-await transformation on js ast.

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

+// 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";

[floitsch, 2015/02/04 17:06:10]

Since you export without a prefix, you should "show" the classes/statics that
you want to see.
The js-library is relatively nasty: it has "number", "string" as top-level
variables.

[sigurdm, 2015/02/05 14:06:07]

Hurray for quick-fixes!

There is a lot of used names.
Maybe it is better to go with a prefix?

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

Hm. probably yes.
In other components it's usally "js" or "jsAst".

+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).
+

[floitsch, 2015/02/04 17:06:10]

Remove empty line.

[sigurdm, 2015/02/05 14:06:07]

Done.

+class AsyncRewriter extends NodeVisitor {

[floitsch, 2015/02/04 17:06:09]

Add comment that you should look at visitFun, visitDartYield, and visitAwait for
comments.

[sigurdm, 2015/02/05 14:06:07]

Done.

+
+  // Local variables are hoisted to the top of the function, so we collect them
+  // 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, so we can ensure 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;
+
+  String resultName;
+  String helperName;
+  String controllerName;
+  String gotoName;
+  String handlerName;
+  String errorName;
+  String nextName;
+  String returnValueName;
+  String outerLabelName;
+  String selfName;
+
+  final Expression thenHelper;

[floitsch, 2015/02/04 17:06:09]

I believe I have written it somewhere else: this is not just a "then"-helper
anymore.

[floitsch, 2015/02/04 17:06:10]

Comments on what these are.

[floitsch, 2015/02/04 17:06:10]

I'm not sure it's worth to have only one variable. With the exception of one
occurrence we know in what function-type we are (sync*, async or async*).

It would be more readable to have different fields.

Potentially even create a BaseRewriter, and then only overload the functions
that have anything to do with async.
That would also allow to have better documentation, since the functions don't
need to explain their implementation for all 3 different cases.

[sigurdm, 2015/02/05 14:06:06]

Done.

[sigurdm, 2015/02/05 14:06:07]

Acknowledged.

[sigurdm, 2015/02/05 14:06:07]

Good idea - I also split the "controller" to a controller and a completer.

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

The more I think about this, I think that we should create three different
rewriters (with one base class). In a future CL. (I think I will come see you in
person...)

+  final Expression newController;
+  final Expression endOfIteration;
+  final Expression newIterable;
+  final Expression yieldExpression;
+  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.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");
+    controllerName = freshName("completer");
+    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() :
+        new LiteralNumber("${errorHandlerLabels.last}");
+  }
+
+  int allocateTempVar() {
+    assert(tempVarHighWaterMark >= currentTempVarIndex);
+    currentTempVarIndex++;
+    tempVarHighWaterMark = max(currentTempVarIndex, tempVarHighWaterMark);
+    return currentTempVarIndex;
+  }
+
+  VariableUse useTempVar(int i) {
+    return tempVarNames.putIfAbsent(i, () =>
+        new VariableUse(freshName("temp$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;
+  }
+
+  /// We collect all the pieces in this map, and output 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 we use a label to break to the 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 will end 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), new LiteralNumber("$label")));
+  }
+
+  /// Return a block that has a goto to [label] including the break.
+  ///
+  /// Will also insert 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));
+    statements.add(breakStatement());
+    return new Block(statements);
+  }
+
+  /// Add a goto to [label] including the break.
+  ///
+  /// Will also insert 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 we can guarantee
+  /// has 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)` we store the first expression in a temporary
+  /// variable.
+  ///
+  /// This is because we need to ensure evaluating node1 before visiting node2,
+  /// 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)` we store 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.
+  ///
+  /// Returning from an async method calls the helper with the result.
+  /// (the result might been stored in [returnValueName] by some finally block).

[floitsch, 2015/02/04 17:06:09]

might have been

[sigurdm, 2015/02/05 14:06:06]

Done.

+  ///
+  /// Returning from a sync* will return a [endOfIteration] marker.

[floitsch, 2015/02/04 17:06:10]

Returning from a sync* returns an [endOfIteration] marker.

[sigurdm, 2015/02/05 14:06:06]

Done.

+  void addReturn() {

[floitsch, 2015/02/04 17:06:09]

Comments don't mention async*

[sigurdm, 2015/02/05 14:06:06]

Done.

+    if (analysis.hasExplicitReturns || isAsyncStar) {
+      beginLabel(returnLabel);
+    } else {
+      addStatement(new Comment("implicit return"));
+    }
+    switch (async) {

[floitsch, 2015/02/04 17:06:10]

How do these guarantee that we go through the finallies?

For example:
foo() sync* {
  try {
    return;
  } finally {
    yield 499;
  }
}

There are even cases, where the finally stops the return:

foo() sync* {
  while (true) {
    try {
      return;
    } finally {
      yield 499;
      continue;
    }
  }
}

If you don't handle these cases yet, just add a test and fix it in a later CL.

[sigurdm, 2015/02/05 14:06:06]

This only generates the return that all returns will eventually jump to.
visitReturn generates the jump that goes through the finally blocks.

I tried to improve the comment to clarify.

+      case const AsyncModifier.async():
+        String returnValue = analysis.hasExplicitReturns
+            ? returnValueName
+            : "null";
+        addStatement(js.statement(
+            "return #thenHelper($returnValue, null, $controllerName, null)",
+            {"thenHelper": thenHelper}));
+        break;
+      case const AsyncModifier.syncStar():
+        addStatement(new Return(new Call(endOfIteration, [])));
+        break;
+      case const AsyncModifier.asyncStar():
+        addStatement(js.statement(
+            "return #thenHelper(null, null, $controllerName, null)",
+            {"thenHelper": thenHelper}));
+        break;
+      default:
+        throw "Internal error, unexpected asyncmodifier";
+    }
+  }
+
+  /// Initializing a controller. (This is a Completer for async,

[floitsch, 2015/02/04 17:06:10]

Initializes the controller/completer.

[sigurdm, 2015/02/05 14:06:07]

The comment here was wrong - I have tried to rewrite.

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

better.

+  /// a StreamController for async* and nothing for a sync*).
+  /// The controller is be passed to each call to [thenHelper].
+  Statement generateInitializer() {
+    switch (async) {
+      case const AsyncModifier.async():
+        return js.statement(
+            "return #thenHelper(null, $helperName, $controllerName, null);",
+                     {"thenHelper": thenHelper});
+      case const AsyncModifier.syncStar():
+        throw "Not handled here";
+      case const AsyncModifier.asyncStar():
+        return js.statement(
+            "return #thenHelper(null, $helperName, $controllerName, null);",
+                     {"thenHelper": thenHelper});      default:
+        throw "Internal error, unexpected asyncmodifier";
+    }
+  }
+
+  @override
+  visitFun(Fun node) {

[floitsch, 2015/02/04 17:06:09]

Add long comment on how you translate from one to the other. With (simplified)
examples.

[sigurdm, 2015/02/05 14:06:06]

Done.

+    if (isSync) return node;
+
+    beginLabel(newLabel("Function start"));
+    // AsyncStar needs a returnlabel for its handling of cancelation. See
+    // [visitYield].

[floitsch, 2015/02/04 12:31:30]

visitDartYield

[sigurdm, 2015/02/05 14:06:07]

Done.

+    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(
+          new LiteralNumber("$label"),
+          new Block(labelledParts[label]));
+    }).toList();
+    Statement helperBody = new Switch(new VariableUse(gotoName), clauses);
+    if (hasJumpThoughOuterLabel) {
+      helperBody = js.statement("$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, new LiteralNumber("0")));
+    if (isAsync || isAsyncStar) {
+      inits.add(makeInit(controllerName, newController));
+    }
+    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,

[floitsch, 2015/02/04 17:06:10]

move "params" into the next line or align the others.

[sigurdm, 2015/02/05 14:06:07]

Done.

+      "helperBody": helperBody,

[floitsch, 2015/02/04 17:06:10]

In a follow-up CL: rename this to something else "helper" is overused.

(I would have also expected that the one in js_helper is the "helper"...)

[sigurdm, 2015/02/05 14:06:07]

Acknowledged.

+      "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
+  visitAccess(PropertyAccess node) {
+    return withExpression2(
+        node.receiver,
+        node.selector,
+        (receiver, selector) => new PropertyAccess(receiver, selector));
+  }
+
+  @override
+  visitArrayHole(ArrayHole node) {
+    return node;
+  }
+
+  @override
+  visitArrayInitializer(ArrayInitializer node) {
+    return withExpressions(node.elements, (elements) {
+      return new ArrayInitializer(elements);
+    });
+  }
+
+  @override
+  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";
+    }
+  }
+
+  @override
+  Expression visitAwait(Await node) {

[floitsch, 2015/02/04 17:06:09]

Add comment explaining what you do here.
Ideally with an example (including the switch around it).

[sigurdm, 2015/02/05 14:06:07]

I refer to the visitFun documentation.

+    int afterWait = newLabel("returning from await.");
+    withExpression(node.expression, (Expression expression) {
+      addStatement(setGotoVariable(afterWait));
+      if (errorHandlerLabels.isEmpty) {
+        addStatement(js.statement("""
+            return #thenHelper(#expression, $helperName, $controllerName, null);
+            """, {"thenHelper": thenHelper, "expression": expression}));
+      } else {
+        addStatement(js.statement("""
+            return #thenHelper(#expression, $helperName, $controllerName,
+                function($errorName) {
+              $gotoName = #currentHandler;
+              $helperName($errorName);
+            });""", {"thenHelper": thenHelper,
+                     "expression": expression,
+                     "currentHandler": currentErrorHandler}));
+      }
+    }, store: false);
+    beginLabel(afterWait);
+    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], we can avoid reduntdantly assigning it
+  /// 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
+  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
+  visitCase(Case node) {
+    return unreachable(node);
+  }
+
+  @override
+  visitCatch(Catch node) {
+    return unreachable(node);
+  }
+
+  @override
+  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,
+                  new LiteralNumber("$thenLabel"),
+                  new LiteralNumber("$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
+  visitContinue(Continue node) {
+    Node target = analysis.targets[node];
+    if (!shouldTransform(target)) {
+      addStatement(node);
+      return;
+    }
+    translateJump(target, continueLabels[target]);
+  }
+
+  /// Returns a break statement that exits the big switch statement.
+  Statement breakStatement() {
+    if (insideUntranslatedBreakable) {
+      hasJumpThoughOuterLabel = true;
+      return new Break(outerLabelName);
+    } else {
+      return new Break(null);
+    }
+  }
+
+  /// Convenience wrapper around [breakStatement].
+  void addBreak() {
+    addStatement(breakStatement());
+  }
+
+  /// Common code for handling break, continue, return.
+  ///
+  /// It is necessary to run all nesting finally-handlers between the jump and
+  /// the target. For that we use [nextName] as a stack of places to go.
+  void translateJump(Node target, int targetLabel) {
+    // Compute a stack of all the 'finally' nodes we must visit 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();
+    // We jump directly to the top of the stack, so take it off now.
+    int firstTarget = jumpStack.removeLast();
+    if (jumpStack.isNotEmpty) {
+      hasJumpThroughFinally = true;
+      Expression jsJumpStack = new ArrayInitializer(
+          jumpStack.map((int label) => new LiteralNumber("$label"))
+                      .toList());
+      addStatement(js.statement("$nextName = #", [jsJumpStack]));
+    }
+    addGoto(firstTarget);
+  }
+
+  @override
+  visitDefault(Default node) => unreachable(node);
+
+  @override
+  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
+  visitEmptyStatement(EmptyStatement node) => node;
+
+  visitExpressionInStatementContext(Expression node) {
+    if (node is VariableDeclarationList) {
+      // Treat VariableDeclarationList as a statement.
+      visitVariableDeclarationList(node);
+    } else {
+      visitExpressionIgnoreResult(node);
+    }
+  }
+
+  @override
+  visitExpressionStatement(ExpressionStatement node) {
+    visitExpressionInStatementContext(node.expression);
+  }
+
+  @override
+  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. We don't have to check.
+      withExpression(node.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
+  visitForIn(ForIn node) {
+    // The dart output currently never uses for-in loops.
+    throw "Javascript for-in not implemented yet in the await transformation";
+  }
+
+  @override
+  visitFunctionDeclaration(FunctionDeclaration node) {
+    return 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
+  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,
+                  new LiteralNumber("$thenLabel"),
+                  new LiteralNumber("$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
+  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
+  visitLiteralBool(LiteralBool node) => node;
+
+  @override
+  visitLiteralExpression(LiteralExpression node) => unsupported(node);
+
+  @override
+  visitLiteralNull(LiteralNull node) => node;
+
+  @override
+  visitLiteralNumber(LiteralNumber node) => node;
+
+  @override
+  visitLiteralStatement(LiteralStatement node) => unsupported(node);
+
+  @override
+  visitLiteralString(LiteralString node) => node;
+
+  @override
+  visitNamedFunction(NamedFunction node) {
+    throw new UnimplementedError("Internal error: "
+        "Named functions are not handled yet by the javascript async "
+        "transform");
+  }
+
+  @override
+  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
+  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
+  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
+  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 ${node.argument}";
+      }
+    }
+    return withExpression(
+        node.argument,
+        (Expression argument) => new Prefix(node.op, argument),
+        store: false);
+  }
+
+  @override
+  visitProgram(Program node) => throw "Unsupported";
+
+  @override
+  visitProperty(Property node) {
+    return withExpression(
+        node.value,
+        (Expression value) => new Property(node.name, value),
+        store: false);
+  }
+
+  @override
+  visitRegExpLiteral(RegExpLiteral node) => node;
+
+  @override
+  visitReturn(Return node) {
+    assert(node.value == null || !isSyncStar && !isAsyncStar);
+    Node target = analysis.targets[node];
+    if (node.value != null) {
+      withExpression(node.value, (Expression value) {
+        addStatement(js.statement("$returnValueName = #", [value]));
+      }, store: false);
+    }
+    translateJump(target, returnLabel);
+  }
+
+  @override
+  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, we have to use a chain of ifs.
+
+      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 $clause");
+        }
+        i++;
+      }
+      withExpression(node.key, (Expression key) {
+        addStatement(new Switch(key, clauses));
+      }, store: false);
+      if (!hasDefault) {
+        addGoto(after);
+      }
+    }
+
+    targetsAndTries.add(node);
+    for (int i = 0; i < labels.length; i++) {
+      beginLabel(labels[i]);
+      visitStatement(node.cases[i].body);
+    }
+    beginLabel(after);
+    targetsAndTries.removeLast();
+  }
+
+  @override
+  visitThis(This node) {
+    return new VariableUse(selfName);
+  }
+
+  @override
+  visitThrow(Throw node) {
+    withExpression(node.expression, (Expression expression) {
+      addStatement(new Throw(expression));
+    }, store: false);
+  }
+
+  setHandler() {
+    addExpressionStatement(
+        new Assignment(new VariableUse(handlerName), currentErrorHandler));
+  }
+
+  @override
+  void visitTry(Try node) {
+    if (!shouldTransform(node)) {
+      Block body = translateInBlock(node.body);
+      Catch catchPart = (node.catchPart == null)
+          ? null
+          : new Catch(node.catchPart.declaration,
+                    translateInBlock(node.catchPart.body));
+      Block finallyPart = (node.finallyPart == null)
+          ? null
+          : translateInBlock(node.finallyPart);
+      addStatement(new Try(body, catchPart, finallyPart));
+      return;
+    }
+    hasTryBlocks = true;
+    int handlerLabel = newLabel("catch");
+    int finallyLabel = newLabel("finally");
+    int afterFinallyLabel = newLabel("after finally");
+    errorHandlerLabels.add(handlerLabel);
+    // We set the error handler here. It must be cleared on every path out.
+    // Normal and error exit.
+    setHandler();
+    if (node.finallyPart != null) {
+      finallyLabels[node] = finallyLabel;
+      targetsAndTries.add(node);
+    }
+    visitStatement(node.body);
+    errorHandlerLabels.removeLast();
+    setHandler();
+    addStatement(js.statement("$nextName = [#];",
+        [new LiteralNumber("$afterFinallyLabel")]));
+    if (node.finallyPart != null) {
+      addGoto(finallyLabel);
+    } else {
+      addGoto(afterFinallyLabel);
+    }
+    beginLabel(handlerLabel);
+    if (node.catchPart != null) {
+      setHandler();
+      // The catch declaration name can shadow outer variables, so we need
+      // a fresh name to avoid collisions.
+      String errorRename = freshName(node.catchPart.declaration.name);
+      localVariables.add(new VariableDeclaration(errorRename));
+      variableRenamings.add(
+          new Pair(node.catchPart.declaration.name, errorRename));
+      addExpressionStatement(
+          new Assignment(new VariableUse(errorRename),
+                         new VariableUse(resultName)));
+      visitStatement(node.catchPart.body);
+      variableRenamings.removeLast();
+    }
+    if (node.finallyPart != null) {
+      targetsAndTries.removeLast();
+      // This belongs to the catch-part, but is only needed if there is a
+      // finally therefore it is in this branch.
+      addStatement(js.statement("$nextName = [#];",
+          [new LiteralNumber("$afterFinallyLabel")]));
+      beginLabel(finallyLabel);
+      visitStatement(node.finallyPart);
+      addStatement(new Comment("// goto the next finally handler"));
+      addStatement(js.statement("$gotoName = $nextName.pop();"));
+      addBreak();
+    }
+    beginLabel(afterFinallyLabel);
+  }
+
+  @override
+  visitVariableDeclaration(VariableDeclaration node) {
+    throw "TODO";
+  }
+
+  @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
+  visitVariableInitialization(VariableInitialization node) {
+    // These should be handled by visitVariableDeclarationList.
+    throw "Unexpected variableInitialization";
+  }
+
+  @override
+  visitVariableUse(VariableUse node) {
+    String renaming = variableRenamings.lastWhere(
+        (Pair renaming) => renaming.a == node.name,
+        orElse: () => new Pair(null, null)).b;
+    if (renaming == null) return node;
+    return new VariableUse(renaming);
+  }
+
+  @override
+  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`, we don't need a test.
+    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);
+  }
+
+  @override
+  visitDartYield(DartYield node) {

[floitsch, 2015/02/04 17:06:10]

Add doc what's happening here.
Ideally with examples.

[sigurdm, 2015/02/05 14:06:07]

Done - no examples though

+    assert(isSyncStar || isAsyncStar);
+    if (isSyncStar) {
+      int label = newLabel("after yield");
+      withExpression(node.expression, (Expression expression) {
+        // We don't do a break here for the goto, but instead a return below.
+        addStatement(setGotoVariable(label));
+        if (node.hasStar) {
+          addStatement(new Return(new Call(yieldStarExpression, [expression])));
+        } else {
+          addStatement(new Return(expression));
+        }
+        beginLabel(label);
+      }, store: false);
+    } else {
+      int label = newLabel("after yield");
+      withExpression(node.expression, (Expression expression) {
+        // We don't do a break here for the goto, but instead a return below.
+        addStatement(setGotoVariable(label));
+        // 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];
+        enclosingFinallyLabels.addAll(targetsAndTries
+            .where((Node node) => node is Try)
+            .map((Try node) => finallyLabels[node]));
+        int destinationOnCancel = enclosingFinallyLabels.removeLast();
+        addStatement(js.statement("""
+            return #thenHelper(#yieldExpression(#expression),
+                $helperName, $controllerName, function () {
+                  if (#notEmptyFinallyList) {

[floitsch, 2015/02/04 17:06:10]

remove {}.
(It's stupid, but otherwise they stay in the output).

[sigurdm, 2015/02/05 14:06:07]

Done.

+                    $nextName = #finallyList;
+                  }
+                  $gotoName = #destinationOnCancel;
+                  $helperName();
+                });""",
+            {"thenHelper": thenHelper,
+             "yieldExpression": node.hasStar
+                  ? yieldStarExpression
+                  : yieldExpression,
+             "expression": expression,
+             "notEmptyFinallyList": enclosingFinallyLabels.isNotEmpty,
+             "finallyList": new ArrayInitializer(enclosingFinallyLabels.map(
+                 (int label) => new LiteralNumber("$label")).toList()),

[floitsch, 2015/02/04 17:06:10]

js.number(label)

[sigurdm, 2015/02/05 14:06:07]

Done (here and elsewhere)

+             "destinationOnCancel": new LiteralNumber("$destinationOnCancel")

[floitsch, 2015/02/04 17:06:10]

js.number(destinationOnCancel)

[sigurdm, 2015/02/05 14:06:07]

Done.

+            }));
+        beginLabel(label);
+      }, store: false);
+    }
+  }
+}
+
+/// 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(this.unsupported);
+
+  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(targets[node] is Loop || targets[node] is LabeledStatement);
+    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;
+  }
+}