Replace ReceivePorts with asyncStart|End in some tests.

tests/lib/async/catch_errors15_test.dart

Index: tests/lib/async/catch_errors15_test.dart
diff --git a/tests/lib/async/catch_errors15_test.dart b/tests/lib/async/catch_errors15_test.dart
index 2cea6feec6c8a7d660200d133e47616fc92f7b47..e76a070849d3303775ab21e870166fef86e7ff7d 100644
--- a/tests/lib/async/catch_errors15_test.dart
+++ b/tests/lib/async/catch_errors15_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 that the outer `catchErrors` waits for the nested `catchErrors` stream
   // to be done.
@@ -46,7 +44,7 @@ main() {
                          "inner done",
                          ],
                          events);
-      port.close();
+      asyncEnd();
     });
   events.add("main exit");
 }