Switch scroll physics over to using newton

sky/sdk/lib/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 'dart:async';
-
-import '../base/scheduler.dart' as scheduler;
-import 'mechanics.dart';
-
-abstract class Generator {
-  Stream<double> get onTick; // TODO(ianh): rename this to tickStream
-  void cancel();
-}
-
-class FrameGenerator extends Generator {
-  Function onDone;
-  StreamController _controller;
-
-  Stream<double> get onTick => _controller.stream;
-
-  int _animationId = 0;
-  bool _cancelled = false;
-
-  FrameGenerator({this.onDone}) {
-    _controller = new StreamController(
-      sync: true,
-      onListen: _scheduleTick,
-      onCancel: cancel);
-  }
-
-  void cancel() {
-    if (_cancelled) {
-      return;
-    }
-    if (_animationId != 0) {
-      scheduler.cancelAnimationFrame(_animationId);
-    }
-    _animationId = 0;
-    _cancelled = true;
-    if (onDone != null) {
-      onDone();
-    }
-  }
-
-  void _scheduleTick() {
-    assert(_animationId == 0);
-    _animationId = scheduler.requestAnimationFrame(_tick);
-  }
-
-  void _tick(double timeStamp) {
-    _animationId = 0;
-    _controller.add(timeStamp);
-    if (!_cancelled) {
-      _scheduleTick();
-    }
-  }
-}
-
-class Simulation extends Generator {
-  Stream<double> get onTick => _stream;
-  final System system;
-
-  FrameGenerator _generator;
-  Stream<double> _stream;
-  double _previousTime = 0.0;
-
-  Simulation(this.system, {Function terminationCondition, Function onDone}) {
-    _generator = new FrameGenerator(onDone: onDone);
-    _stream = _generator.onTick.map(_update);
-
-    if (terminationCondition != null) {
-      bool done = false;
-      _stream = _stream.takeWhile((_) {
-        if (done)
-          return false;
-        done = terminationCondition();
-        return true;
-      });
-    }
-  }
-
-  void cancel() {
-    _generator.cancel();
-  }
-
-  double _update(double timeStamp) {
-    double previousTime = _previousTime;
-    _previousTime = timeStamp;
-    if (previousTime == 0.0)
-      return timeStamp;
-    double deltaT = timeStamp - previousTime;
-    system.update(deltaT);
-    return timeStamp;
-  }
-}