Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/run_zoned2_test.dart

Index: tests/lib/async/run_zoned2_test.dart
diff --git a/tests/lib/async/run_zoned2_test.dart b/tests/lib/async/run_zoned2_test.dart
index 39e28a09815226922a8b50fe4f464095edfa6c20..fa9110f978f74cfe4456fc0682d8bee157ee8301 100644
--- a/tests/lib/async/run_zoned2_test.dart
+++ b/tests/lib/async/run_zoned2_test.dart
@@ -2,15 +2,13 @@
 // 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 finishes.
   runZonedExperimental(() => 499,
-                       onDone: port.close);
+                       onDone: asyncEnd);
 }