anchor.diff:  part of skysprites;
   
  +typedef void PhysicsJointBreakCallback(PhysicsJoint joint);
  +
   abstract class PhysicsJoint {
  -  PhysicsJoint(this._bodyA, this._bodyB, this.breakingForce) {
  +  PhysicsJoint(this._bodyA, this._bodyB, this.breakingForce, this.breakCallback) {
       bodyA._joints.add(this);
       bodyB._joints.add(this);
     }
  
positive.code:     PhysicsBody bodyA,
      PhysicsBody bodyB, {
        double breakingForce,
        this.dampening: 0.0,
        this.frequency: 0.0
      }
    ) : super(bodyA, bodyB, breakingForce) {
      _completeCreation();
    }
  
  
negative.code:     PhysicsBody bodyA,
      PhysicsBody bodyB, {
        double breakingForce,
        PhysicsJointBreakCallback breakCallback,
        this.ratio: 0.0
      }
    ) : super(bodyA, bodyB, breakingForce, breakCallback) {
      _completeCreation();
    }
  
  