Switch to async_helper package.

tests/isolate/global_error_handler_stream2_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() {
   IsolateSink mainIsolate;
   stream.listen((msg) {
     mainIsolate = msg;
     throw new UnsupportedError("ignore exception");
   }, onDone: () {
     mainIsolate.add("received done");
     mainIsolate.close();
   });
@@ skipping 23 matching lines @@
     asyncEnd();
   });
   asyncStart();
   box.stream.single.then((msg) {
     Expect.equals("received done", msg);
     timer.cancel();
     keepRunningBox.stream.close();
     asyncEnd();
   });
 }