Fix two bugs with async stack traces.

tests/language/await_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=10 --no-background-compilation
+// VMOptions=--optimization-counter-threshold=10 --no-background-compilation --stress-async-stacks
 
 import 'package:expect/expect.dart';
 
 import 'dart:async';
 
 int globalVariable = 1;
 int topLevelFoo(int param) => 1;
 int get topLevelGetter => globalVariable;
 void set topLevelSetter(val) {
   globalVariable = val;
@@ skipping 107 matching lines @@
 main() {
   for (int i = 0; i < 10; i++) {
     staticMembers();
     topLevelMembers();
     instanceMembers();
     conditionals();
     others();
   }
 }