fix analyzer warnings in stocks app

sky/sdk/lib/app/view.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:sky' as sky;
 
 import '../rendering/box.dart';
 import '../rendering/object.dart';
 import 'scheduler.dart' as scheduler;
 
@@ skipping 37 matching lines @@
 
   ViewConstraints get _viewConstraints =>
       new ViewConstraints(width: sky.view.width, height: sky.view.height);
 
   Map<int, PointerState> _stateForPointer = new Map<int, PointerState>();
 
   Function onFrame;
 
   final List<EventListener> _eventListeners = new List<EventListener>();
   void addEventListener(EventListener e) => _eventListeners.add(e);
-  void removeEventListener(EventListener e) => _eventListeners.remove(e);
+  bool removeEventListener(EventListener e) => _eventListeners.remove(e);
 
   RenderBox get root => _renderView.child;
   void set root(RenderBox value) {
     _renderView.child = value;
   }
   void _beginFrame(double timeStamp) {
     if (onFrame != null)
       onFrame();
     RenderObject.flushLayout();
     _renderView.paintFrame();
@@ skipping 58 matching lines @@
       entry.target.handleEvent(event, entry);
   }
 
   String toString() => 'Render Tree:\n${_renderView}';
 
   void debugDumpRenderTree() {
     toString().split('\n').forEach(print);
   }
   
 }