Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/intercept_run_async4_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';
 
 var events = [];
 
 body() {
   events.add("body entry");
   runAsync(() {
     events.add("run async body");
     runAsync(() {
       events.add("run nested body");
     });
   });
   return 499;
 }
 
 handler(fun) {
   events.add("handler");
   runAsync(fun);
   events.add("handler 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 body of a runAsync goes to the zone it came from.
   var result = runZonedExperimental(body, onRunAsync: handler);
   events.add("after");
   runAsync(() {
     runAsync(() {
       Expect.listEquals(
           ["body entry",
            "handler", "handler done",
            "after",
            "run async body",
            "handler", "handler done",
            "run nested body"],
           events);
-      port.close();
+      asyncEnd();
     });
   });
 }