Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/intercept_run_async6_test.dart

Index: tests/lib/async/intercept_run_async6_test.dart
diff --git a/tests/lib/async/intercept_run_async6_test.dart b/tests/lib/async/intercept_run_async6_test.dart
index 8b49edf78947424a217b0f7397e9951fb9394751..82cfcd0884056002030bd619f97c0c596c5ff5e4 100644
--- a/tests/lib/async/intercept_run_async6_test.dart
+++ b/tests/lib/async/intercept_run_async6_test.dart
@@ -2,9 +2,9 @@
 // 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';
 
 class A {
   add(x) => print(x);
@@ -35,9 +35,7 @@ onDoneHandler() {
 }
 
 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();
 
   // Test that runZonedExperimental works when async, error and done are used.
   var result = runZonedExperimental(
@@ -55,6 +53,6 @@ main() {
          "error: foo",
          "done"],
         events);
-    port.close();
+    asyncEnd();
  });
 }