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:   Ticker _ticker;
  
    Simulation _simulation;
    Duration _startTime;
  
    double _value = 0.0;
    /// The current value of the simulation
  
negative.code:     assert(simulation != null);
      assert(!_ticker.isTicking);
      _simulation = simulation;
      _value = simulation.x(0.0);
      return _ticker.start();
    }
  