Added async_helper and fixed broken asynchronous test.

tests/lib/async/run_async6_test.dart

 // Copyright (c) 2013, 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 run_async_test;
 
 import 'dart:async';
 import '../../../pkg/unittest/lib/unittest.dart';
 
 bool _unhandledExceptionCallback(exception) => true;
 
 main() {
   test('run async test', () {
-    var callback = expectAsync0(() {});
-    runAsync(() {
-      runAsync(() {
-        callback();
-      });
+    runAsync(expectAsync0({
+      runAsync(expectAsync0(() {}));

[floitsch, 2013/06/20 14:59:35]

why is this change needed?

[kustermann, 2013/06/25 07:14:21]

You're right, this should not be necessary.

       throw new Exception('exception');
-    });
+    }));
   });
 }