Rationalize Dart mojo and sky package structure

sky/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);
-}
-
-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() {
-    sky.window.cancelAnimationFrame(_animationId);
-    _startTime = 0.0;
-    _duration = 0.0;
-    _animationId = 0;
-  }
-
-  void _scheduleTick() {
-    assert(_animationId == 0);
-    _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);
-  }
-}