Migrated test block 168 to Dart 2.0.

tests/lib_2/async/event_helper.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 event_helper;
 
 import 'dart:async';
 
 abstract class Event {
   void replay(EventSink sink);
@@ skipping 164 matching lines @@
   void resume() {
     if (trace) print("Events#$hashCode: resume");
     subscription.resume();
   }
 
   void onDone(void action()) {
     if (trace) print("Events#$hashCode: onDone");
     super.onDone(action);
   }
 }