Switch to async_helper package.

tests/isolate/illegal_msg_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 illegal_msg_tests;
 import "package:expect/expect.dart";
 import 'dart:isolate';
-import '../async_helper.dart';
+import "package:async_helper/async_helper.dart";
 
 funcFoo(x) => x + 2;
 
 echo() {
   port.receive((msg, reply) {
     reply.send("echoing ${msg(1)}}");
   });
 }
 
 main() {
   var snd = spawnFunction(echo);
   var caught_exception = false;
   try {
     snd.send(funcFoo, port.toSendPort());
   } catch (e) {
     caught_exception = true;
   }
 
   if (caught_exception) {
     port.close();
   } else {
     asyncStart();
     port.receive((msg, reply) {
       print("from worker ${msg}");
       asyncEnd();
     });
   }
   Expect.isTrue(caught_exception);
 }