Fix parent pointer issue in async rewriter.

lib/transformations/continuation.dart

Index: lib/transformations/continuation.dart
diff --git a/lib/transformations/continuation.dart b/lib/transformations/continuation.dart
index 233da291346b3fe5877c5a7b6ee71c4946bf659f..751e2ac365ddb01d3fabb5056c34ea5a25093776 100644
--- a/lib/transformations/continuation.dart
+++ b/lib/transformations/continuation.dart
@@ -499,7 +499,8 @@ abstract class AsyncRewriterBase extends ContinuationRewriterBase {
     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)));