Fix various warnings from the analyzer.

tests/lib/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';
 
-class Event {
+abstract class Event {
   void replay(StreamSink sink);
 }
 
 class DataEvent implements Event {
   final data;
 
   DataEvent(this.data);
 
   void replay(StreamSink sink) { sink.add(data); }
 
@@ skipping 94 matching lines @@
    */
   void pause([Future resumeSignal]) {
     throw new StateError("Not capturing events.");
   }
 
   /** Resumes after a call to [pause]. */
   void resume() {
     throw new StateError("Not capturing events.");
   }
 
-  /** Whether the underlying subscription has been paused. */
-  bool get isPaused => false;
-
   /**
    * Sets an action to be called when this [Events] receives a 'done' event.
    */
   void onDone(void action()) {
      throw new StateError("Not capturing events.");
   }
 }
 
 class CaptureEvents extends Events {
   StreamSubscription subscription;
@@ skipping 21 matching lines @@
   }
 
   void pause([Future resumeSignal]) {
     subscription.pause(resumeSignal);
   }
 
   void resume() {
     subscription.resume();
   }
 
-  bool get isPaused => subscription.isPaused;
-
   void onDone(void action()) {
     onDoneSignal.future.whenComplete(action);
   }
 }