anchor.diff:  
     PhysicsBodyType type;
   
  -  double density;
  -  double friction;
  -  double restitution;
  -  bool isSensor;
  +  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;
  
positive.code:   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,
      this.linearVelocity: Offset.zero,
      this.angularVelocity: 0.0,
      this.linearDampening: 0.0,
  
negative.code:     this.bullet: false,
      this.active: true,
      this.gravityScale: 1.0
    }) {
      this.density = density;
      this.friction = friction;
      this.restitution = restitution;
      this.isSensor = isSensor;
    }
  
    Object tag;
  
  