Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/run_zoned7_test.dart

Index: tests/lib/async/run_zoned7_test.dart
diff --git a/tests/lib/async/run_zoned7_test.dart b/tests/lib/async/run_zoned7_test.dart
index 3bd3dbbbec9f0ec869c9f2ca77a11f824c079c6f..e6f4067ce0acdc98bf678d0d1c4dec43d54082b4 100644
--- a/tests/lib/async/run_zoned7_test.dart
+++ b/tests/lib/async/run_zoned7_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';
 import 'catch_errors.dart';
 
 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();
   var events = [];
   // Test runZoned with periodic Timers.
   runZonedExperimental(() {
@@ -26,7 +24,7 @@ main() {
                          1, 2, 3, 4, 5, 6,
                          ],
                          events);
-      port.close();
+      asyncEnd();
     });
   events.add("main exit");
 }