Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/future_delayed_error_test.dart

 // Copyright (c) 2012, 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.
 
 library future_delayed_error_test;
 
+import 'package:async_helper/async_helper.dart';
 import "package:expect/expect.dart";
 import 'dart:async';
-import 'dart:isolate';
 
 testImmediateError() {
-  // An open ReceivePort keeps the VM running. If the error-handler below is not
-  // executed then the test will fail with a timeout.
-  var port = new ReceivePort();
+  asyncStart();
   var future = new Future.error("error");
   future.catchError((error) {
-    port.close();
     Expect.equals(error, "error");
+    asyncEnd();
   });
 }
 
 Future get completedFuture {
   var completer = new Completer();
   completer.completeError("foobar");
   return completer.future;
 }
 
 testDelayedError() {
-  var port = new ReceivePort();
+  asyncStart();
   completedFuture.catchError((error) {
-    port.close();
     Expect.equals(error, "foobar");
+    asyncEnd();
   });
 }
 
 main() {
   testImmediateError();
   testDelayedError();
 }