anchor.diff:    PhysicsBody(this.shape, {
       this.tag: null,
       this.type: PhysicsBodyType.dynamic,
  -    this.density: 1.0,
  -    this.friction: 0.0,
  -    this.restitution: 0.0,
  -    this.isSensor: false,
  +    double density: 1.0,
  +    double friction: 0.0,
  +    double restitution: 0.0,
  +    bool isSensor: false,
       this.linearVelocity: Offset.zero,
       this.angularVelocity: 0.0,
       this.linearDampening: 0.0,
  
positive.code:     this.bullet: false,
      this.active: true,
      this.gravityScale: 1.0
    });
  
    Object tag;
  
  
negative.code: 
    PhysicsBodyType type;
  
    double _density;
  
    double get density => _density;
  
    set density(double density) {
      _density = density;
  
      if (_body == null)
        return;
      for(box2d.Fixture f = _body.getFixtureList(); f != null; f = f.getNext()) {
        f.setDensity(density);
      }
    }
  
    double _friction;
  
    double get friction => _friction;
  
    set friction(double friction) {
      _friction = friction;
  
      if (_body == null)
        return;
      for(box2d.Fixture f = _body.getFixtureList(); f != null; f = f.getNext()) {
        f.setFriction(friction);
      }
    }
  
    double _restitution;
  
    double get restitution => _restitution;
  
    set restitution(double restitution) {
      _restitution = restitution;
  
      if (_body == null)
        return;
      for(box2d.Fixture f = _body.getFixtureList(); f != null; f = f.getNext()) {
        f.setRestitution(restitution);
      }
    }
  
    bool _isSensor;
  
    bool get isSensor => _isSensor;
  
    set isSensor(bool isSensor) {
      _isSensor = isSensor;
  
      if (_body == null)
        return;
      for(box2d.Fixture f = _body.getFixtureList(); f != null; f = f.getNext()) {
        f.setSensor(isSensor);
      }
    }
  
    Offset linearVelocity;
    double angularVelocity;
  