anchor.diff:      bodyB._joints.add(this);
     }
   
  -  PhysicsBody bodyA;
  -  PhysicsBody bodyB;
  +  final PhysicsBody bodyA;
  +  final PhysicsBody bodyB;
   
     bool _active = true;
     box2d.Joint _joint;
  
positive.code:   PhysicsJointPrismatic(
      PhysicsBody bodyA,
      PhysicsBody bodyB,
      this.axis) : super(bodyA, bodyB) {
      _completeCreation();
    }
  
  
negative.code: class PhysicsJointWeld extends PhysicsJoint {
    PhysicsJointWeld(
      PhysicsBody bodyA,
      PhysicsBody bodyB
    ) : super(bodyA, bodyB) {
      _completeCreation();
    }
  
  