anchor.diff:  
   /// A curve that is 0.0 until it hits the threshold, then it jumps to 1.0.
   ///
  -/// ![](https://flutter.github.io/assets-for-api-docs/assets/animation/curve_threshold.png)
  +/// {@animation 464 192 https://flutter.github.io/assets-for-api-docs/assets/animation/curve_threshold.mp4}
   class Threshold extends Curve {
     /// Creates a threshold curve.
     ///
  
positive.code: /// An instance of this class using the default period of 0.4 is available as
  /// [Curves.elasticOut].
  ///
  /// ![](https://flutter.github.io/assets-for-api-docs/assets/animation/curve_elastic_out.png)
  class ElasticOutCurve extends Curve {
    /// Creates an elastic-out curve.
    ///
  
negative.code: /// set to 1.0 will essentially become a three-second animation that starts
  /// three seconds later.
  ///
  /// {@animation 464 192 https://flutter.github.io/assets-for-api-docs/assets/animation/curve_interval.mp4}
  class Interval extends Curve {
    /// Creates an interval curve.
    ///
  