Switch to async_helper package.

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';
+import "package:async_helper/async_helper.dart";
 
 runTest() {
   SendPort mainIsolate;
   bool isFirst = true;
   port.receive((msg, replyTo) {
     if (isFirst) {
       mainIsolate = msg;
       isFirst = false;
       throw new UnsupportedError("ignore exception");
     }
@@ skipping 15 matching lines @@
   otherIsolate.send(port.toSendPort());
   otherIsolate.send("message 2");
   asyncStart();
   port.receive((msg, replyPort) {
     Expect.equals("received", msg);
     port.close();
     timer.cancel();
     asyncEnd();
   });
 }