Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/run_zoned7_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';
 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(() {
     int counter = 0;
     new Timer.periodic(const Duration(milliseconds: 50), (timer) {
       if (counter == 5) timer.cancel();
       counter++;
       events.add(counter);
     });
   }, onDone: () {
       Expect.listEquals([
                          "main exit",
                          1, 2, 3, 4, 5, 6,
                          ],
                          events);
-      port.close();
+      asyncEnd();
     });
   events.add("main exit");
 }