anchor.diff:    Ticker _ticker;
   
     Simulation _simulation;
  -  Duration _startTime;
   
     double _value = 0.0;
     /// The current value of the simulation
  
positive.code: 
    Completer _completer;
    int _animationId;
  
    /// Start calling onTick once per animation frame
    ///
    /// The returned future resolves once the ticker stops ticking.
    Future start() {
      assert(!isTicking);
      _completer = new Completer();
      _scheduleTick();
      return _completer.future;
  
negative.code: import 'package:newton/newton.dart';
  import 'package:sky/src/animation/scheduler.dart';
  
  typedef _TickerCallback(Duration elapsed);
  
  /// Calls its callback once per animation frame
  class Ticker {
  