Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/elements/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" as sky;
 
 abstract class AnimationDelegate {
   void updateAnimation(double t);
 }
@@ skipping 31 matching lines @@
       _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);
   }
 }