anchor.diff:    PhysicsJointWeld(
       PhysicsBody bodyA,
       PhysicsBody bodyB, {
  -      double breakingForce
  +      double breakingForce,
  +      this.dampening: 0.0,
  +      this.frequency: 0.0
       }
     ) : super(bodyA, bodyB, breakingForce) {
       _completeCreation();
     }
  +
  +  final double dampening;
  +  final double frequency;
   
     box2d.Joint _createB2Joint(PhysicsNode physicsNode) {
       box2d.WeldJointDef b2Def = new box2d.WeldJointDef();
  
positive.code:       (bodyA._body.position.y + bodyB._body.position.y) / 2.0
      );
      b2Def.initialize(bodyA._body, bodyB._body, middle);
      return physicsNode.b2World.createJoint(b2Def);
    }
  }
  
negative.code:   PhysicsJointRevolute(
      PhysicsBody bodyA,
      PhysicsBody bodyB,
      this._worldAnchor, {
        this.lowerAngle: 0.0,
        this.upperAngle: 0.0,
        this.enableLimit: false,
  