anchor.diff:  
       if (_body == null)
         return;
  -    for(box2d.Fixture f = _body.getFixtureList(); f != null; f = f.getNext()) {
  +    for (box2d.Fixture f = _body.getFixtureList(); f != null; f = f.getNext()) {
         f.setFriction(friction);
       }
     }
  
positive.code: 
      if (_body == null)
        return;
      for(box2d.Fixture f = _body.getFixtureList(); f != null; f = f.getNext()) {
        f.setSensor(isSensor);
      }
    }
  
    Offset linearVelocity;
    double angularVelocity;
  
    double linearDampening;
  
    double angularDampening;
  
    bool allowSleep;
  
  
negative.code: 
      if (_body == null)
        return;
      for (box2d.Fixture f = _body.getFixtureList(); f != null; f = f.getNext()) {
        f.setDensity(density);
      }
    }
  