anchor.diff:  import 'package:newton/newton.dart';
   import 'package:sky/src/animation/scheduler.dart';
   
  -typedef _TickerCallback(Duration timeStamp);
  +typedef _TickerCallback(Duration elapsed);
   
   /// Calls its callback once per animation frame
   class Ticker {
  
positive.code:   /// Stop ticking the current simulation
    void stop() {
      _simulation = null;
      _startTime = null;
      _ticker.stop();
    }
  
    /// Whether this object is currently ticking a simulation
    bool get isAnimating => _ticker.isTicking;
  
    void _tick(Duration timeStamp) {
      if (_startTime == null)
        _startTime = timeStamp;
  
      double timeInMicroseconds = (timeStamp - _startTime).inMicroseconds.toDouble();
      double timeInSeconds =  timeInMicroseconds / Duration.MICROSECONDS_PER_SECOND;
      _value = _simulation.x(timeInSeconds);
      final bool isLastTick = _simulation.isDone(timeInSeconds);
  
      if (isLastTick)
        stop();
  
      _onTick(_value);
  
negative.code:   Ticker _ticker;
  
    Simulation _simulation;
  
    double _value = 0.0;
    /// The current value of the simulation
  