Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/run_zoned3_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` is done when a synchronous call throws.
   bool sawException = false;
   try {
     runZonedExperimental(() { throw 0; },
                          onDone: () {
                            // onDone is executed synchronously.
                            Expect.isFalse(sawException);
-                           port.close();
+                           asyncEnd();
                          });
   } catch (e) {
     sawException = true;
   }
   Expect.isTrue(sawException);
 }