Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/catch_errors20_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 that nested stream (from `catchErrors`) that is delayed by a future
   // is waited for.
   catchErrors(() {
     catchErrors(() {
       new Future.error(499);
       new Future.delayed(const Duration(milliseconds: 20), () {
         events.add(42);
       });
     }).listen(events.add,
               onDone: () { events.add("done"); });
     throw "foo";
   }).listen((x) { events.add("outer: $x"); },
             onDone: () {
               Expect.listEquals(["outer: foo",
                                  499,
                                  42,
                                  "done",
                                 ],
                                 events);
-              port.close();
+              asyncEnd();
             });
 }