Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/animation/generators.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 'curves.dart';
 import 'dart:async';
 import 'dart:math' as math;
 import 'dart:sky' as sky;
 import 'mechanics.dart';
 
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   double _update(double timeStamp) {
     double previousTime = _previousTime;
     _previousTime = timeStamp;
     if (previousTime == 0.0)
       return timeStamp;
     double deltaT = timeStamp - previousTime;
     system.update(deltaT);
     return timeStamp;
   }
 }