anchor.diff:        _joint.getReactionForce(1.0 / dt, reactionForce);
   
         if (breakingForce * breakingForce < reactionForce.length2) {
  -        // TODO: Add callback
  -
  +        // Destroy the joint
           destroy();
  +
  +        // Notify any observer
  +        if (breakCallback != null)
  +          breakCallback(this);
         }
       }
     }
  
positive.code:     PhysicsBody bodyA,
      PhysicsBody bodyB,
      this.axis, {
        double breakingForce
      }
    ) : super(bodyA, bodyB, breakingForce) {
      _completeCreation();
    }
  
  
negative.code:     this.anchorA,
      this.anchorB, {
        double breakingForce,
        PhysicsJointBreakCallback breakCallback,
        this.length,
        this.dampening: 0.0,
        this.frequency: 0.0
      }
    ) : super(bodyA, bodyB, breakingForce, breakCallback) {
      _completeCreation();
    }
  
  