anchor.diff:      PhysicsBody bodyA,
       PhysicsBody bodyB, {
         double breakingForce,
  +      PhysicsJointBreakCallback breakCallback,
         this.dampening: 0.0,
         this.frequency: 0.0
       }
  -  ) : super(bodyA, bodyB, breakingForce) {
  +  ) : super(bodyA, bodyB, breakingForce, breakCallback) {
       _completeCreation();
     }
   
  
positive.code:   PhysicsBody get bodyB => _bodyB;
  
    final double breakingForce;
  
    bool _active = true;
    box2d.Joint _joint;
  
negative.code:     PhysicsBody bodyB,
      this.anchor, {
        double breakingForce,
        PhysicsJointBreakCallback breakCallback,
        this.maxForce: 0.0,
        this.maxTorque: 0.0
      }
    ) : super(bodyA, bodyB, breakingForce, breakCallback) {
      _completeCreation();
    }
  
  