anchor.diff:  
   const double kTwoPi = 2 * math.PI;
   
  -class SectorConstraints {
  +class SectorConstraints extends Constraints {
     const SectorConstraints({
       this.minDeltaRadius: 0.0,
       this.maxDeltaRadius: double.INFINITY,
  
positive.code:   double constrainDeltaTheta(double deltaTheta) {
      return clamp(min: minDeltaTheta, max: maxDeltaTheta, value: deltaTheta);
    }
  }
  
  class SectorDimensions {
  
negative.code:     return new SectorDimensions.withConstraints(constraints);
    }
  
    SectorConstraints get constraints => super.constraints;
    void performResize() {
      // default behaviour for subclasses that have sizedByParent = true
      deltaRadius = constraints.constrainDeltaRadius(0.0);
  