Fix parent pointer issue in async rewriter.

lib/transformations/continuation.dart

 // Copyright (c) 2016, 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 kernel.transformations.continuation;
 
 import 'dart:math' as math;
 
 import '../ast.dart';
 import '../visitor.dart';
@@ skipping 481 matching lines @@
 
     LabeledStatement labeled = new LabeledStatement(null);
     if (cond != null) {
       loopBody.addAll(condEffects);
     } else {
       cond = new BoolLiteral(true);
     }
     loopBody.add(
         new IfStatement(cond, new Block(newBody), new BreakStatement(labeled)));
     labeled.body =
-        new WhileStatement(new BoolLiteral(true), new Block(loopBody));
+        new WhileStatement(new BoolLiteral(true), new Block(loopBody))
+        ..parent = labeled;
     statements.add(new Block(<Statement>[]
       ..addAll(temps)
       ..add(labeled)));
     return null;
   }
 
   TreeNode visitForInStatement(ForInStatement stmt) {
     if (stmt.isAsync) {
       // Transform
       //
@@ skipping 376 matching lines @@
         findFactoryConstructor(completerClass, 'sync'),
         findConstructor(syncIterableClass, ''),
         findConstructor(streamIteratorClass, ''),
         findFactoryConstructor(futureClass, 'microtask'),
         findConstructor(streamControllerClass, ''),
         findProcedure(asyncLibrary, '_asyncThenWrapperHelper'),
         findProcedure(asyncLibrary, '_asyncErrorWrapperHelper'),
         findProcedure(asyncLibrary, '_awaitHelper'));
   }
 }