Switch to async_helper package.

tests/isolate/illegal_msg_stream_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.
 
 library test;
 
 import 'package:expect/expect.dart';
 import 'dart:isolate';
-import '../async_helper.dart';
+import "package:async_helper/async_helper.dart";
 
 funcFoo(x) => x + 2;
 
 foo() {
   stream.single.then((msg) {
     IsolateSink sink = msg;
     sink.add(499);
     sink.close();
   });
 }
 
 main() {
   var box = new MessageBox();
   var snd = streamSpawnFunction(foo);
   var caught_exception = false;
   try {
     snd.add(funcFoo);
   } catch (e) {
     caught_exception = true;
   }
   snd.add(box.sink);
   snd.close();
 
   asyncStart();
   box.stream.single.then((msg) {
     Expect.equals(499, msg);
     asyncEnd();
   });
 }