anchor.diff:  }
   
   /// A curve that is 0.0 until start, then curved from 0.0 to 1.0 at end, then 1.0.
  -class Interval implements Curve {
  +class Interval extends Curve {
     const Interval(this.start, this.end, { this.curve: Curves.linear });
   
     /// The smallest value for which this interval is 0.0.
  
positive.code:   return 7.5625 * t * t + 0.984375;
  }
  
  /// An oscillating curve that grows in magnitude.
  class BounceInCurve implements Curve {
    const BounceInCurve();
    double transform(double t) {
      return 1.0 - _bounce(1.0 - t);
  
negative.code: }
  
  /// An oscillating curve that shrink in magnitude.
  class BounceOutCurve extends Curve {
    const BounceOutCurve();
    double transform(double t) {
      return _bounce(t);
  