Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/run_zoned5_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';
 
 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();
   // Ensure that `runZoned`'s onError handles synchronous errors.
   runZonedExperimental(() { throw 0; },
                        onError: (e) {
                          Expect.equals(0, e);
-                         port.close();
+                         asyncEnd();
                        });
 }