Fix two bugs with async stack traces.

tests/language/await_future_test.dart

 // Copyright (c) 2014, 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.
 
 // VMOptions=--optimization-counter-threshold=5 --no-background-compilation
+// VMOptions=--optimization-counter-threshold=5 --no-background-compilation --stress-async-stacks
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 
 import 'dart:async';
 
 // It does not matter where a future is generated.
 bar(p) async => p;
 baz(p) => new Future(() => p);
 
@@ skipping 210 matching lines @@
     result = await awaitForIn();
     Expect.equals('abc', result);
   }
 }
 
 main() {
   asyncStart();
   test().then((_) {
     asyncEnd();
   });
-}
+}