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