Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/intercept_run_async6_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.
 
+import 'package:async_helper/async_helper.dart';
 import "package:expect/expect.dart";
 import 'dart:async';
-import 'dart:isolate';
 
 class A {
   add(x) => print(x);
 }
 var events = [];
 
 body() {
   events.add("body entry");
   runAsync(() {
     events.add("run async body");
@@ skipping 10 matching lines @@
 
 onErrorHandler(e) {
   events.add("error: $e");
 }
 
 onDoneHandler() {
   events.add("done");
 }
 
 main() {
-  // We keep a ReceivePort open until all tests are done. This way the VM will
-  // hang if the callbacks are not invoked and the test will time out.
-  var port = new ReceivePort();
+  asyncStart();
 
   // Test that runZonedExperimental works when async, error and done are used.
   var result = runZonedExperimental(
       body,
       onRunAsync: onAsyncHandler,
       onError: onErrorHandler,
       onDone: onDoneHandler);
   events.add("after");
   Timer.run(() {
     Expect.listEquals(
         ["body entry",
          "async handler", "async handler done",
          "after",
          "run async body",
          "error: foo",
          "done"],
         events);
-    port.close();
+    asyncEnd();
  });
 }