Create the addEventListener API

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;
 
 class PointerState {
   HitTestResult result;
   Point lastPosition;
 
   PointerState({ this.result, this.lastPosition });
 }
 
+typedef void EventListener(sky.Event event);
+
 class AppView {
 
   AppView({ RenderBox root: null, RenderView renderViewOverride }) {
     assert(_app == null);
     _app = this;
 
     sky.view.setEventCallback(_handleEvent);
     sky.view.setMetricsChangedCallback(_handleMetricsChanged);
     scheduler.init();
     scheduler.addPersistentFrameCallback(_beginFrame);
@@ skipping 16 matching lines @@
   RenderView _renderView;
   RenderView get renderView => _renderView;
 
   ViewConstraints get _viewConstraints =>
       new ViewConstraints(width: sky.view.width, height: sky.view.height);
 
   Map<int, PointerState> _stateForPointer = new Map<int, PointerState>();
 
   Function onFrame;
 
-  List<sky.EventListener> eventListeners = new List<sky.EventListener>();
+  final List<EventListener> _eventListeners = new List<EventListener>();
+  void addEventListener(EventListener e) => _eventListeners.add(e);
+  void 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();
   }
 
   void _handleEvent(sky.Event event) {
     if (event is sky.PointerEvent) {
       _handlePointerEvent(event);
     } else if (event is sky.GestureEvent) {
       HitTestResult result = new HitTestResult();
       _renderView.hitTest(result, position: new Point(event.x, event.y));
       dispatchEvent(event, result);
     } else {
-      for (sky.EventListener listener in eventListeners) {
-        listener(event);
+      for (EventListener e in _eventListeners) {
+        e(event);
       }
     }
   }
 
   void _handleMetricsChanged() {
     _renderView.rootConstraints = _viewConstraints;
   }
 
   PointerState _createStateForPointer(sky.PointerEvent event, Point position) {
     HitTestResult result = new HitTestResult();
@@ skipping 36 matching lines @@
       entry.target.handleEvent(event, entry);
   }
 
   String toString() => 'Render Tree:\n${_renderView}';
 
   void debugDumpRenderTree() {
     toString().split('\n').forEach(print);
   }
   
 }