Added async_helper and fixed broken asynchronous test.

tests/isolate/isolate2_negative_test.dart

 // Copyright (c) 2012, 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.
 
 // Dart test program for testing that exceptions in other isolates bring down
 // the program.
 
 library isolate2_negative_test;
 import 'dart:isolate';
+import '../async_helper.dart';
 
 void entry() {
   throw "foo";
 }
 
 main() {
+  // We start an asynchronous operation, but since we don't expect to get
+  // anything back except an exception there is no asyncEnd().
+  // If the exception is not thrown this test will timeout.
+  asyncStart();
   SendPort port = spawnFunction(entry);
 }