anchor.diff:      }
     }
   
  +  void run() {
  +    window.animationFrame.then(update);
  +  }
  +
     void update(num delta) {
       final num diff = delta - _lastTimestamp;
   
       if (diff > GAME_SPEED) {
         _lastTimestamp = delta;
  -      clear();
  +      _clear();
         drawCell(_ctx, _food, "blue");
         _snake.update(_ctx, _keyboard);
         _checkForCollisions();
  
positive.code:   }
  
    bool isPressed(int keyCode) => _keys.containsKey(keyCode);
  }
negative.code:   }
  
    void _move() {
      // add a new head segment
      grow();
  
      // remove the tail segment
  