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;
+
+import "js.dart";
+import "../helpers/helpers.dart";
+import 'dart:collection';
+import "dart:math" show max;
+import 'package:compiler/src/util/util.dart';
+
+class AsyncFinder extends BaseVisitor {
+  @override
+  visitFun(Fun node) {
+    switch (node.asyncModifier) {
+      case const AsyncModifier.sync():
+        node.visitChildren(this);
+        break;
+      case const AsyncModifier.async():
+
+        node.visitChildren(this);
+        break;
+      case const AsyncModifier.syncStar():
+        throw "Sync* is not supported yet";
+      case const AsyncModifier.asyncStar():
+        throw "Async* is not supported yet";
+    }
+  }
+
+}
+
+class AnalysisPrinter extends BaseVisitor {
+  int indent = 0;
+  var awaits;
+  visitNode(Node node) {
+    indent ++;
+    node.visitChildren(this);
+    indent --;
+  }
+}
+
+class Analysis extends NodeVisitor {
+  Set<Node> awaits = new Set<Node>();
+
+  Map<Node, Node> targets = new Map<Node, Node>();
+  List<Node> loopsAndSwitches = new List<Node>();
+  List<LabeledStatement> labelledStatements = new List<LabeledStatement>();
+
+  bool visit(Node node) {
+    bool result = node.accept(this);

[floitsch, 2015/01/20 16:06:15]

don't call it "result".

[sigurdm, 2015/02/02 10:20:15]

Called it containsAwait

+    if (result) {
+      awaits.add(node);
+    }
+    return result;
+  }
+
+  analyze(Fun node) {
+    visit(node.body);
+  }
+
+  @override
+  visitAccess(PropertyAccess node) {
+    bool x = visit(node.receiver);
+    bool y = visit(node.selector);
+    return x || y;
+  }
+
+  @override
+  visitArrayHole(ArrayHole node) {
+    // TODO: implement visitArrayHole

[floitsch, 2015/01/20 16:06:15]

should be trivial. There is no expression in the hole, and it doesn't have a
side-effect.

[sigurdm, 2015/02/02 10:20:14]

Done.

+  }
+
+  @override
+  visitArrayInitializer(ArrayInitializer node) {
+    // TODO: implement visitArrayInitializer
+  }
+
+  @override
+  visitAssignment(Assignment node) {
+    bool x = visit(node.leftHandSide);

[floitsch, 2015/01/20 16:06:15]

x -> left
y -> right

[sigurdm, 2015/02/02 10:20:15]

Done.

+    bool y = node.value == null
+        ? false
+        : visit(node.value);
+    return x || y;
+  }
+
+  @override
+  visitAwait(Await node) {
+    visit(node.expression);
+    return true;
+  }
+
+  @override
+  visitBinary(Binary node) {
+    bool x = visit(node.left);
+    bool y = visit(node.right);
+    return x || y;
+  }
+
+  @override
+  visitBlob(Blob node) {
+    return false;
+  }
+
+  @override
+  visitBlock(Block node) {
+    bool result = false;
+    for (var s in node.statements) {
+      if (visit(s)) result = true;
+    }
+    return result;
+  }
+
+  @override
+  visitBreak(Break node) {
+    if (node.targetLabel != null) {
+      targets[node] = labelledStatements.lastWhere(
+          (LabeledStatement stm) => stm.label == node.targetLabel);
+    } else {
+      targets[node] = loopsAndSwitches.last;
+    }
+    return false;
+  }
+
+  @override
+  visitCall(Call node) {
+    bool result = visit(node.target);
+    for (var s in node.arguments) {
+      if (visit(s)) result = true;
+    }
+    return result;
+  }
+
+  @override
+  visitCase(Case node) {
+    bool x = visit(node.expression);
+    bool y = visit(node.body);
+    return x || y;
+  }
+
+  @override
+  visitCatch(Catch node) {
+    bool x = visit(node.declaration);
+    bool y = visit(node.body);
+    return x || y;
+  }
+
+  @override
+  visitComment(Comment node) {
+    return false;
+  }
+
+  @override
+  visitConditional(Conditional node) {
+    bool x = visit(node.condition);
+    bool y = visit(node.then);
+    bool z = visit(node.otherwise);
+    return x || y || z;
+  }
+
+  @override
+  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);
+    }
+    return false;
+  }
+
+  @override
+  visitDefault(Default node) {
+    return visit(node.body);
+  }
+
+  @override
+  visitDo(Do node) {
+    loopsAndSwitches.add(node);
+    bool x = visit(node.body);
+    bool y = visit(node.condition);
+    loopsAndSwitches.removeLast();
+    return x || y;
+  }
+
+  @override
+  visitEmptyStatement(EmptyStatement node) {
+    return false;
+  }
+
+  @override
+  visitExpressionStatement(ExpressionStatement node) {
+    return visit(node.expression);
+  }
+
+  @override
+  visitFor(For node) {
+    loopsAndSwitches.add(node);

[floitsch, 2015/01/20 16:06:15]

I would add the loopsAndSwitches only for the body.
It doesn't make a difference, since the init, condition and update can not have
a break/continue anyways, but still.

[sigurdm, 2015/02/02 10:20:15]

Good point

Done

+    bool x = visit(node.init);
+    bool y = visit(node.condition);
+    bool z = visit(node.update);
+    bool u = visit(node.body);
+    loopsAndSwitches.removeLast();
+    return x || y || z || u;
+  }
+
+  @override
+  visitForIn(ForIn node) {
+    loopsAndSwitches.add(node);
+    loopsAndSwitches.removeLast();
+    // TODO: implement visitForIn
+    throw "BADDD";
+  }
+
+  @override
+  visitFun(Fun node) {
+    return false;
+  }
+
+  @override
+  visitFunctionDeclaration(FunctionDeclaration node) {
+    return false;
+  }
+
+  @override
+  visitIf(If node) {
+    bool x = visit(node.condition);
+    bool y = visit(node.then);
+    bool z = visit(node.otherwise);
+    return x || y || z;
+  }
+
+  @override
+  visitInterpolatedExpression(InterpolatedExpression node) {
+    // TODO: implement visitInterpolatedExpression
+  }
+
+  @override
+  visitInterpolatedLiteral(InterpolatedLiteral node) {
+    // TODO: implement visitInterpolatedLiteral
+  }
+
+  @override
+  visitInterpolatedParameter(InterpolatedParameter node) {
+    // TODO: implement visitInterpolatedParameter
+  }
+
+  @override
+  visitInterpolatedSelector(InterpolatedSelector node) {
+    // TODO: implement visitInterpolatedSelector
+  }
+
+  @override
+  visitInterpolatedStatement(InterpolatedStatement node) {
+    // TODO: implement visitInterpolatedStatement
+  }
+
+  @override
+  visitLabeledStatement(LabeledStatement node) {
+    labelledStatements.add(node);
+    bool result = visit(node.body);
+    labelledStatements.removeLast();
+    return result;
+  }
+
+  @override
+  visitLiteralBool(LiteralBool node) {
+    return false;
+  }
+
+  @override
+  visitLiteralExpression(LiteralExpression node) {
+    // TODO: implement visitLiteralExpression
+  }
+
+  @override
+  visitLiteralNull(LiteralNull node) {
+    return false;
+  }
+
+  @override
+  visitLiteralNumber(LiteralNumber node) {
+    return false;
+  }
+
+  @override
+  visitLiteralStatement(LiteralStatement node) {
+    // TODO: implement visitLiteralStatement
+  }
+
+  @override
+  visitLiteralString(LiteralString node) {
+    return false;
+  }
+
+  @override
+  visitNamedFunction(NamedFunction node) {
+    return false;
+  }
+
+  @override
+  visitNew(New node) {
+    return visitCall(node);
+  }
+
+  @override
+  visitObjectInitializer(ObjectInitializer node) {
+    bool result = false;
+    for (Property property in node.properties) {
+      if (visit(property)) result = true;
+    }
+    return result;
+  }
+
+  @override
+  visitParameter(Parameter node) {
+  }
+
+  @override
+  visitPostfix(Postfix node) {
+    return visit(node.argument);
+  }
+
+  @override
+  visitPrefix(Prefix node) {
+    return visit(node.argument);
+  }
+
+  @override
+  visitProgram(Program node) {
+    throw "Unexpected";
+  }
+
+  @override
+  visitProperty(Property node) {
+    return visit(node.value);
+  }
+
+  @override
+  visitRegExpLiteral(RegExpLiteral node) {
+    return false;
+  }
+
+  @override
+  visitReturn(Return node) {
+    if (node.value == null) return false;
+    return visit(node.value);
+  }
+
+  @override
+  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
+  visitThis(This node) {
+    return false;
+  }
+
+  @override
+  visitThrow(Throw node) {
+    return visit(node.expression);
+  }
+
+  @override
+  visitTry(Try node) {
+    bool x = visit(node.body);
+    bool y = node.catchPart == null ? false : visit(node.catchPart);
+    bool z = node.finallyPart == null ? false : visit(node.finallyPart);
+    return x || y || z;
+  }
+
+  @override
+  visitVariableDeclaration(VariableDeclaration node) {
+    return false;
+  }
+
+  @override
+  visitVariableDeclarationList(VariableDeclarationList node) {
+    bool result = false;
+    for (VariableInitialization init in node.declarations) {
+      if (visit(init)) result = true;
+    }
+    return result;
+  }
+
+  @override
+  visitVariableInitialization(VariableInitialization node) {
+    return visitAssignment(node);
+  }
+
+  @override
+  visitVariableUse(VariableUse node) {
+    return false;
+  }
+
+  @override
+  visitWhile(While node) {
+    loopsAndSwitches.add(node);
+    // Be careful to avoid short-circuit.
+    bool x = visit(node.condition);
+    bool y = visit(node.body);
+    loopsAndSwitches.removeLast();
+    return x || y;
+  }
+}
+
+class AsyncRewriter extends NodeVisitor {
+
+  // The set of all nodes containing an await nested somewhere.
+  Set<Node> __containsAwait;
+
+  // Local variables are hoisted to the top of the function, so we collect them
+  // here.
+  List<VariableDeclaration> localVariables = new List<VariableDeclaration>();
+
+  Map<Node, Node> __targets = new Map<Node, Node>();
+  Map<Node, int> continueLabels = new Map<Node, int>();
+  Map<Node, int> breakLabels = new Map<Node, int>();
+
+  List<Pair<String, String>> variableRenamings =
+      new List<Pair<String, String>>();
+  Set<String> usedNames = new Set<String>();
+
+  List<int> errorHandlerLabels = new List<int>();
+
+  // TODO(sigurdm): use namer for these.
+  String resultName = "__result";
+  String helperName = "__helper";
+  String gotoName = "__goto";
+  String handlerName = "__handler";
+  String errorName = "__error";
+
+  int __currentLabel = 0;
+
+  int highWaterMark = 0;
+  int currentTempVarIndex = 0;
+
+  int allocateTempVar() {
+    assert(highWaterMark >= currentTempVarIndex);
+    currentTempVarIndex++;
+    highWaterMark = max(currentTempVarIndex, highWaterMark);
+    return currentTempVarIndex;
+  }
+
+  deallocateTempVar([howMany = 1]) {
+    currentTempVarIndex -= howMany;
+  }
+
+  Expression useTempVar(int i) {
+    return new VariableUse("__temp$i");
+  }
+
+
+  String freshName(String originalName) {
+    String result = "__$originalName";
+    int counter = 1;
+    while (usedNames.contains(result)) {
+      result = "__$originalName$counter";
+      ++counter;
+    }
+    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.

[floitsch, 2015/01/20 16:06:15]

New line before.

[sigurdm, 2015/02/02 10:20:15]

Done.

+  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 tryBlocks = false;
+
+  List<Statement> currentStatementBuffer;
+
+  void addStatement(Statement node) {
+    currentStatementBuffer.add(node);
+  }
+
+  void addExpressionStatement(Expression node) {
+    addStatement(new ExpressionStatement(node));
+  }
+
+  Expression methodCall(Expression callee,
+                        String methodName,
+                        List<Expression> args) {
+    return new Call(new PropertyAccess.field(callee, methodName), args);
+  }
+
+  int newLabel([String comment]) {
+    int result = __currentLabel;
+    __currentLabel++;
+    if (comment != null) {
+      labelComments[result] = comment;
+    }
+    return result;
+  }
+
+  void beginLabel(int label) {
+    assert(!labelledParts.containsKey(label));
+    labelledParts[label] = currentStatementBuffer = new List<Statement>();
+    addStatement(new Comment(labelComments[label]));
+  }
+
+  /// Returns a statement assigning to the `__goto` variable. This should be
+  /// followed by a break for the goto to be executed. Use [gotoWithBreak] or
+  /// [addGoto] for this.
+  Statement goto(int label) {
+    return new ExpressionStatement(
+        new Assignment(new VariableUse(gotoName), new LiteralNumber("$label")));
+  }
+
+  /// Returns a block with the same effect as [addGoto].
+  Block gotoAndBreak(int label) {
+    List<Statement> statements = new List<Statement>();
+    if (labelComments.containsKey(label)) {
+      statements.add(new Comment("goto ${labelComments[label]}"));
+    }
+    statements.add(goto(label));
+    statements.add(new Break(null));
+    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(goto(label));
+    addStatement(new Break(null));
+  }
+
+  bool transform(Node node) {

[floitsch, 2015/01/20 16:06:14]

shouldTransform

[sigurdm, 2015/02/02 10:20:15]

Done.

+    return node is Break || node is Continue || __containsAwait.contains(node);
+  }
+
+  /// Main entry point.
+  Fun rewrite(Fun node) {
+    assert(node.asyncModifier == const AsyncModifier.async());
+    Analysis analysis = new Analysis();
+    analysis.analyze(node);
+    __containsAwait = analysis.awaits;
+    __targets = analysis.targets;
+    new AnalysisPrinter()..awaits = analysis.awaits
+        ..visitNode(node);
+    return node.accept(this);
+  }
+
+  visitStatement(Statement node) {
+    node.accept(this);
+  }
+
+  void visitExpressionIgnoreResult(Expression node) {
+    addExpressionStatement(node.accept(this));
+  }
+
+  Expression visitExpression(Expression node) {
+    return node.accept(this);
+  }
+
+  Expression _storeIfNecessary(Expression result) {
+    if (result is Literal) return result;

[floitsch, 2015/01/20 16:06:15]

I guess this will be wrong, once you have list-literals that contain
side-effects.

[sigurdm, 2015/02/02 10:20:14]

Actually list-literals are not Literal, but ObjectInitializers and
ArrayInitializers, so I think this will work.
But I will add a comment

+    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;
+  }
+
+  withExpression2(Expression node1, Expression node2,
+                  fn(Expression result1, Expression result2)) {
+    int oldTempVarIndex = currentTempVarIndex;
+    Expression r1 = visitExpression(node1);
+    if (transform(node2)) {
+      r1 = _storeIfNecessary(r1);
+    }
+    Expression r2 = visitExpression(node2);
+    var result = fn(r1, r2);
+    currentTempVarIndex = oldTempVarIndex;
+    return result;
+  }
+
+  withExpressions(List<Node> nodes, fn(List<Node> 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 (transform(nodes[i])) {
+        lastTransformIndex = i;
+        break;
+      }
+    }
+    List<Node> visited = nodes.take(lastTransformIndex).map((Node node) {
+      return _storeIfNecessary(visitExpression(node));
+    }).toList();
+    visited.addAll(nodes.skip(lastTransformIndex).map((Node node) {
+      return visitExpression(node);
+    }));
+    var result = fn(visited);
+    currentTempVarIndex = oldTempVarIndex;
+    return result;
+  }
+
+  // [[Fun(a1, ..., an) S ]] =>
+  //   Fun(a1, ..., an) {
+  //     vars(S); // Declaration of local variables of S.
+  //     var __goto = 0;
+  //     var __handler = null; // The current error handler.
+  //     Fun helper(__result) {
+  //       while(true) {
+  //         try {
+  //           switch(__goto) {
+  //           case 0:
+  //             [[S]];
+  //           }
+  //         } catch(__error) {
+  //           if (handler === null) throw error;
+  //           __goto = __handler;
+  //           __result = __error;
+  //         }
+  //       }
+  //     }
+  //   }
+  visitAsyncFun(Fun node) {
+    beginLabel(newLabel("Function start"));
+    visitStatement(node.body);
+    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 (tryBlocks) {
+      helperBody = new Try(new Block([helperBody]),

[floitsch, 2015/01/20 16:06:15]

consider using the `js` templates.
You will actually end up reusing the template, and it's easier to read.

[sigurdm, 2015/02/02 10:20:14]

Yeah - it is better like that.

+          new Catch((new VariableDeclaration(errorName)),
+              new Block([new If.noElse(new Binary("===",
+                            new VariableUse(handlerName), new LiteralNull()),
+                             new Throw(new VariableUse(errorName))),
+                         new ExpressionStatement(new Assignment(
+                             new VariableUse(resultName),
+                             new VariableUse(errorName))),
+                         new ExpressionStatement(new Assignment(
+                             new VariableUse(gotoName),
+                             new VariableUse(handlerName))),
+                         ])), null);
+    }
+    helperBody = new While(new LiteralBool(true), helperBody);
+    helperBody = new Block([helperBody]);
+    List<VariableInitialization> inits =
+        [new VariableInitialization(new VariableDeclaration(gotoName),
+                                    new LiteralNumber("0")),
+         new VariableInitialization(new VariableDeclaration(handlerName),
+                                    new LiteralNull())];
+    inits.addAll(localVariables.map(
+        (VariableDeclaration decl) => new VariableInitialization(decl, null)));
+    VariableDeclarationList varDecl = new VariableDeclarationList(inits);
+    inits.addAll(new Iterable.generate(highWaterMark, (int i) {
+      return new VariableInitialization(
+          new VariableDeclaration("__temp${i + 1}"),
+          null);
+    }));
+    Fun helper = new Fun([new Parameter(resultName)], helperBody);
+    return new Fun(node.params,
+        new Block([new ExpressionStatement(varDecl),
+                   new FunctionDeclaration(new VariableDeclaration(helperName),
+                                           helper),
+                   new Return(
+                       new New(new PropertyAccess.field(
+                           new VariableUse("Future"), "microtask"),
+                           [new VariableUse(helperName)]))]));
+  }
+
+  @override
+  visitFun(Fun node) {
+    switch (node.asyncModifier) {
+      case const AsyncModifier.sync():
+        return node;
+      case const AsyncModifier.async():
+        return visitAsyncFun(node);
+      case const AsyncModifier.syncStar():
+        throw "Sync* is not supported yet";
+      case const AsyncModifier.asyncStar():
+        throw "Async* is not supported yet";
+    }
+
+  }
+
+  @override
+  visitAccess(PropertyAccess node) {
+    return withExpression2(node.receiver, node.selector,
+        (receiver, selector) => new PropertyAccess(receiver, selector));
+  }
+
+  unreachable(Node node) {
+    throw("The transformer should never meet: $node");

[floitsch, 2015/01/20 16:06:15]

If the function is intendend to stay, make it have a better error-message.
Start with:
throw "Internal error: trying to visit $node";

(If the error is encountered by real users, they don't want to hear that the
node is not unreachable. It clearly is, otherwise they wouldn't see the error).

Eventually it should throw a real exception and/or the element that caused the
issue.

[sigurdm, 2015/02/02 10:20:14]

Done.

+  }
+
+  @override
+  visitArrayHole(ArrayHole node) {
+    return node;
+  }
+
+  @override
+  visitArrayInitializer(ArrayInitializer node) {
+    withExpressions(node.elements, (elements) {
+      return new ArrayInitializer(elements);
+    });
+  }
+
+  // C([[e1 = e2]] =>
+  // C([[e1]] = [[e2]])
+  @override
+  visitAssignment(Assignment node) {
+    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(
+            new PropertyAccess(evaluated[0], evaluated[1]),
+            evaluated[2]);
+      });
+    } else {
+      throw "Unexpected assignment left hand side $leftHandSide";
+    }
+  }
+
+  // C([[await e]]) =>
+  //   __goto = `newLabel(afterCall)
+  //   [[e]].then(__helper, onError: (error) {__goto = `currentHandler } );
+  // case afterCall:
+  //   C(__result);
+  @override
+  Expression visitAwait(Await node) {
+    int afterWait = newLabel("Returning from await.");
+    withExpression(node.expression, (Expression expression) {
+      addStatement(goto(afterWait));
+      addStatement(new Return(new Call(
+          new PropertyAccess.field(expression, "then"),
+          [new VariableUse(helperName),
+           new Fun([new Parameter(errorName)],
+               new Block([new ExpressionStatement(
+                             new Assignment(new VariableUse(gotoName),
+                                            currentErrorHandler)),
+                          new ExpressionStatement(new Call(
+                              new VariableUse(helperName),
+                              [new VariableUse(errorName)]))]))])));
+    }, store: false);
+    beginLabel(afterWait);
+    return new VariableUse(resultName);
+  }
+
+  Expression get currentErrorHandler {
+    return errorHandlerLabels.isEmpty
+        ? new LiteralNull()
+        : new LiteralNumber("${errorHandlerLabels.last}");
+  }
+
+  // TODO short-circuiting operators!!
+  @override
+  Expression visitBinary(Binary node) {
+    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) {
+    int target = breakLabels[__targets[node]];
+    if (target != null) {
+      addGoto(target);
+    } else {
+      addStatement(node);
+    }
+  }
+
+  @override
+  Expression visitCall(Call node) {
+    bool storeTarget = node.arguments.any(transform);
+    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 (!transform(node.then) && !transform(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);
+    addStatement(new Break(null));
+    beginLabel(thenLabel);
+    withExpression(node.then, (Expression value) {
+      addExpressionStatement(new Assignment(
+          new VariableUse(resultName),
+          value));
+    }, store: false);
+    addGoto(joinLabel);
+    beginLabel(elseLabel);
+    withExpression(node.otherwise, (Expression value) {
+      addExpressionStatement(new Assignment(
+          new VariableUse(resultName),
+          value));
+    }, store: false);
+    beginLabel(joinLabel);
+    return new VariableUse(resultName);
+  }
+
+  @override
+  visitContinue(Continue node) {
+    int target = continueLabels[__targets[node]];
+    if (target != null) {
+      addGoto(target);
+    } else {
+      addStatement(node);
+    }
+  }
+
+  @override
+  visitDefault(Default node) => unreachable(node);
+
+  @override
+  visitDo(Do node) {
+    throw "TODO";
+  }
+
+  @override
+  visitEmptyStatement(EmptyStatement node) => node;
+
+  @override
+  visitExpressionStatement(ExpressionStatement node) {
+    if (node.expression is VariableDeclarationList) {
+      // Treat VariableDeclarationList as a statement.
+      visitVariableDeclarationList(node.expression);
+    } else {
+      visitExpressionIgnoreResult(node.expression);
+    }
+  }
+
+  @override
+  visitFor(For node) {
+    throw "TODO";
+  }
+
+  @override
+  visitForIn(ForIn node) {
+    throw "TODO";
+  }
+
+  @override
+  visitFunctionDeclaration(FunctionDeclaration node) {
+    return node;
+  }
+
+  Block translateInBlock(Statement node) {
+    List<Statement> oldBuffer = currentStatementBuffer;
+    List<Statement> resultBuffer = currentStatementBuffer = new List();
+    visitStatement(node);
+    currentStatementBuffer = oldBuffer;
+    return new Block(resultBuffer);
+  }
+
+  @override
+  visitIf(If node) {
+    if (!transform(node.then) && !transform(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);
+    addStatement(new Break(null));
+    beginLabel(thenLabel);
+    visitStatement(node.then);
+    if (node.otherwise is EmptyStatement) {
+      addGoto(joinLabel);
+      beginLabel(elseLabel);
+      visitStatement(node.otherwise);
+    }
+    beginLabel(joinLabel);
+  }
+
+  @override
+  visitInterpolatedExpression(InterpolatedExpression node) {
+    throw "Unsupported";
+  }
+
+  @override
+  visitInterpolatedLiteral(InterpolatedLiteral node) => throw "Unsupported";
+
+  @override
+  visitInterpolatedParameter(InterpolatedParameter node) => throw "Unsupported";
+
+  @override
+  visitInterpolatedSelector(InterpolatedSelector node) => throw "Unsupported";
+
+  @override
+  visitInterpolatedStatement(InterpolatedStatement node) => throw "Unsupported";
+
+  @override
+  visitLabeledStatement(LabeledStatement node) {
+    int breakLabel = newLabel("break ${node.label}");
+    int continueLabel = newLabel("continue ${node.label}");
+    breakLabels[node] = breakLabel;
+    continueLabels[node] = continueLabel;
+
+    beginLabel(continueLabel);
+    visitStatement(node.body);
+    beginLabel(breakLabel);
+  }
+
+  @override
+  visitLiteralBool(LiteralBool node) => node;
+
+  @override
+  visitLiteralExpression(LiteralExpression node) => throw "Unsupported";
+
+  @override
+  visitLiteralNull(LiteralNull node) => node;
+
+  @override
+  visitLiteralNumber(LiteralNumber node) => node;
+
+  @override
+  visitLiteralStatement(LiteralStatement node) => throw "Unsupported";
+
+  @override
+  visitLiteralString(LiteralString node) => node;
+
+  @override
+  visitNamedFunction(NamedFunction node) {
+    throw "TODO";
+  }
+
+  @override
+  visitNew(New node) {
+    bool storeTarget = node.arguments.any(transform);
+    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, (List<Node> properties) {
+      return new ObjectInitializer(properties);
+    });
+  }
+
+  @override
+  visitParameter(Parameter node) {
+    throw "Unexpected";
+  }
+
+  @override
+  visitPostfix(Postfix node) {
+    return withExpression(node.argument,
+        (Expression argument) => new Postfix(node.op, argument), store: false);
+  }
+
+  @override
+  visitPrefix(Prefix node) {
+    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) {
+    if (node.value == null) {
+      addStatement(node);
+    } else {
+      withExpression(node.value, (Expression value) {
+        addStatement(new Return(value));
+      }, store: false);
+    }
+  }
+
+  // [[switch(e) {
+  //   case i: Si;
+  //   ...
+  //   default: Sd;
+  // }]] =>
+  // case #before
+  //   switch([[e]]) {
+  //     case i: __goto = #i;
+  //     ...
+  //     default: __goto = #defaultLabel;
+  //   }
+  // case #i:
+  //   [[Si, breakLabels|#after, continueLabels|#before]]
+  //   // Notice fallthough/
+  // ...
+  // case #defaultLabel:
+  //   [[Si, breakLabels|#after, continueLabels|#before]]
+  // case #afterSwitch:
+  @override
+  visitSwitch(Switch node) {
+    if (!node.cases.any(transform)) {
+      // If only the key has an await, translation can be simplified.
+      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));
+      });
+      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 && transform(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 default case is handled last.
+            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 {
+          throw "Unknown switchclause  $i";
+        }
+        i++;
+      }
+      withExpression(node.key, (Expression key) {
+        addStatement(new Switch(key, clauses));
+      }, store: false);
+      if (!hasDefault) {
+        addGoto(after);
+      }
+    }
+    for (int i = 0; i < labels.length; i++) {
+      beginLabel(labels[i]);
+      visitStatement(node.cases[i].body);
+    }
+    beginLabel(after);
+  }
+
+  @override
+  visitThis(This node) => node;
+
+  @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 (!transform(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;
+    }
+    tryBlocks = true;
+    int handlerLabel = newLabel("catch");
+    int finallyLabel = newLabel("finally");
+    errorHandlerLabels.add(handlerLabel);
+    setHandler();
+    visitStatement(node.body);
+    errorHandlerLabels.removeLast();
+    setHandler();
+    addGoto(finallyLabel);
+    beginLabel(handlerLabel);
+    if (node.catchPart != null) {
+      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();
+    }
+    beginLabel(finallyLabel);
+    if (node.finallyPart != null) {
+      visitStatement(node.finallyPart);
+    }
+  }
+
+  @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: () {
+      return new Pair(null, null);
+    }).b;
+    if (renaming == null) return node;
+    return new VariableUse(renaming);
+  }
+
+  @override
+  visitWhile(While node) {
+    if (!transform(node.body)) {
+      // If only the condition has an await, translation can be simplified.
+      withExpression(node.condition, (Expression condition) {
+        addStatement(new While(condition, translateInBlock(node.body)));
+      }, store: false);
+      return;
+    }
+    int continueLabel = newLabel("while condition");
+    continueLabels[node] = continueLabel;
+    beginLabel(continueLabel);
+
+    int bodyLabel = newLabel("while body");
+    int after = newLabel("after while");
+    breakLabels[node] = after;
+    withExpression(node.condition, (Expression cond) {
+      addExpressionStatement(new Assignment(new VariableUse(gotoName),
+          new Conditional(cond, new LiteralNumber("$bodyLabel"),
+              new LiteralNumber("$after"))));
+    }, store: false);
+    addStatement(new Break(null));
+    beginLabel(bodyLabel);
+    visitStatement(node.body);
+    addGoto(continueLabel);
+    beginLabel(after);
+  }
+}