Rationalize Dart mojo and sky package structure

sky/sdk/lib/framework/animation/mechanics.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;
 
 const double kGravity = -0.980; // m^s-2
 
 abstract class System {
   void update(double deltaT);
@@ skipping 193 matching lines @@
 
   void update(double deltaT) {
     _currentSystem.update(deltaT);
   }
 
   void transitionToSystem(System system) {
     assert(system != null);
     _currentSystem = system;
   }
 }