anchor.diff:      double friction: 0.0,
       double restitution: 0.0,
       bool isSensor: false,
  -    this.linearVelocity: Offset.zero,
  -    this.angularVelocity: 0.0,
  +    Offset linearVelocity: Offset.zero,
  +    double angularVelocity: 0.0,
       this.linearDampening: 0.0,
  -    this.angularDampening: 0.0,
  +    double awakeangularDampening: 0.0,
       this.allowSleep: true,
       this.awake: true,
       this.fixedRotation: false,
  
positive.code: 
      if (_body == null)
        return;
      for(box2d.Fixture f = _body.getFixtureList(); f != null; f = f.getNext()) {
        f.setFriction(friction);
      }
    }
  
negative.code: 
      if (_body == null)
        return;
      for (box2d.Fixture f = _body.getFixtureList(); f != null; f = f.getNext()) {
        f.setDensity(density);
      }
    }
  