Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/catch_errors6_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 completer = new Completer();
   var errorHandlerOrDoneHasBeenExecuted = false;
   // Test that `catchErrors` doesn't shut down if a future is never completed.
   catchErrors(() {
     completer.future.then((x) { Expect.fail("should not be executed"); });
   }).listen((x) {
       errorHandlerOrDoneHasBeenExecuted = true;
       Expect.fail("should not be executed (listen)");
     },
     onDone: () {
       errorHandlerOrDoneHasBeenExecuted = true;
       Expect.fail("should not be executed (onDone)");
     });
   Timer.run(() {
     Expect.isFalse(errorHandlerOrDoneHasBeenExecuted);
-    port.close();
+    asyncEnd();
   });
 }