anchor.diff:    /// Creates an interval curve.
     ///
     /// The [start] and [end] arguments must not be null.
  -  const Interval(this.start, this.end, { this.curve: Curves.linear });
  +  const Interval(this.begin, this.end, { this.curve: Curves.linear });
   
     /// The smallest value for which this interval is 0.0.
  -  final double start;
  +  final double begin;
   
     /// The smallest value for which this interval is 1.0.
     final double end;
  
positive.code:   @override
    String toString() {
      if (curve is! Linear)
        return '$runtimeType($start\u22EF$end)\u27A9$curve';
      return '$runtimeType($start\u22EF$end)';
    }
  }
  
  
negative.code: 
    @override
    double transform(double t) {
      assert(begin >= 0.0);
      assert(begin <= 1.0);
      assert(end >= 0.0);
      assert(end <= 1.0);
      assert(end >= begin);
      if (t == 0.0 || t == 1.0)
        return t;
      t = ((t - begin) / (end - begin)).clamp(0.0, 1.0);
      if (t == 0.0 || t == 1.0)
        return t;
      return curve.transform(t);
  