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:     this.anchor,
      this.axis, {
        double breakingForce,
        this.dampening: 0.0,
        this.frequency: 0.0
      }
    ) : super(bodyA, bodyB, breakingForce) {
      _completeCreation();
    }
  
  
negative.code:       _joint.getReactionForce(1.0 / dt, reactionForce);
  
        if (breakingForce * breakingForce < reactionForce.length2) {
          // Destroy the joint
          destroy();
  
          // Notify any observer
          if (breakCallback != null)
            breakCallback(this);
        }
      }
    }
  