Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/run_zoned8_test.dart

Index: tests/lib/async/run_zoned8_test.dart
diff --git a/tests/lib/async/run_zoned8_test.dart b/tests/lib/async/run_zoned8_test.dart
index c8ce77d9b1ea2bd4b74676453f0ba945cf96c34c..6c0c89f3a116d26364d2bf332822c0f37015214c 100644
--- a/tests/lib/async/run_zoned8_test.dart
+++ b/tests/lib/async/run_zoned8_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(() {
@@ -30,7 +28,7 @@ main() {
                            1, "error: 1", 2, "error: 2",
                            ],
                            events);
-         port.close();
+         asyncEnd();
        });
   events.add("main exit");
 }