anchor.diff:      bodyB._joints.add(this);
     }
   
  -  PhysicsBody bodyA;
  -  PhysicsBody bodyB;
  +  final PhysicsBody bodyA;
  +  final PhysicsBody bodyB;
   
     bool _active = true;
     box2d.Joint _joint;
  
positive.code:     PhysicsBody bodyA,
      PhysicsBody bodyB,
      this.worldAnchor, {
        double lowerAngle: 0.0,
        double upperAngle: 0.0,
        bool enableLimit: false
      }) : super(bodyA, bodyB) {
      this.lowerAngle = lowerAngle;
      this.upperAngle = upperAngle;
      this.enableLimit = enableLimit;
  
      _completeCreation();
    }
  
    Point worldAnchor;
    double lowerAngle;
    double upperAngle;
    bool enableLimit;
  
    box2d.Joint _createB2Joint(PhysicsNode physicsNode) {
      // Create Joint Definition
  
negative.code:   PhysicsJointPrismatic(
      PhysicsBody bodyA,
      PhysicsBody bodyB,
      this.axis
    ) : super(bodyA, bodyB) {
      _completeCreation();
    }
  
  