Fix errors in Sky detected by Dart analyzer

sky/framework/animation/timer.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:math" as math;
-import "dart:sky";
+import "dart:sky" as sky;
 
 abstract class AnimationDelegate {
   void updateAnimation(double t);
 }
 
 class AnimationTimer {
   final AnimationDelegate _delegate;
   double _startTime = 0.0;
   double _duration = 0.0;
   int _animationId = 0;
 
   AnimationTimer(this._delegate);
 
   void start(double duration) {
     if (_animationId != 0)
       stop();
     _duration = duration;
     _scheduleTick();
   }
 
   void stop() {
-    window.cancelAnimationFrame(_animationId);
+    sky.window.cancelAnimationFrame(_animationId);
     _startTime = 0.0;
     _duration = 0.0;
     _animationId = 0;
   }
 
   void _scheduleTick() {
     assert(_animationId == 0);
-    _animationId = window.requestAnimationFrame(_tick);
+    _animationId = sky.window.requestAnimationFrame(_tick);
   }
 
   void _tick(double timeStamp) {
     _animationId = 0;
     if (_startTime == 0.0)
       _startTime = timeStamp;
     double elapsedTime = timeStamp - _startTime;
     double t = math.max(0.0, math.min(1.0, elapsedTime / _duration));
     if (t < 1.0)
       _scheduleTick();
     else
       stop();
     _delegate.updateAnimation(t);
   }
 }