Added async_helper and fixed broken asynchronous test.

tests/isolate/global_error_handler2_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 test;
 
 import 'package:expect/expect.dart';
 import 'dart:async';
 import 'dart:isolate';
+import '../async_helper.dart';
 
 runTest() {
   SendPort mainIsolate;
   bool isFirst = true;
   port.receive((msg, replyTo) {
     if (isFirst) {
       mainIsolate = msg;
       isFirst = false;
       throw new UnsupportedError("ignore exception");
     }
     Expect.equals("message 2", msg);
     mainIsolate.send("received");
   });
 }
 
 bool globalErrorHandler(IsolateUnhandledException e) {
   return e.source is UnsupportedError && e.source.message == "ignore exception";
 }
 
 main() {
   // Make sure this test doesn't last longer than 2 seconds.
   var timer = new Timer(const Duration(seconds: 2), () { throw "failed"; });
 
   var port = new ReceivePort();
   SendPort otherIsolate = spawnFunction(runTest, globalErrorHandler);
   otherIsolate.send(port.toSendPort());
   otherIsolate.send("message 2");
+  asyncStart();
   port.receive((msg, replyPort) {
     Expect.equals("received", msg);
     port.close();
     timer.cancel();
+    asyncEnd();
   });
 }