anchor.diff:      _food = _randomPoint();
     }
   
  -  void run() {
  -    window.animationFrame.then(update);
  -  }
  -
  -  void clear() {
  +  void _clear() {
       _ctx..fillStyle = "white"
         ..fillRect(0, 0, _canvas.width, _canvas.height);
     }
  
positive.code:     }
    }
  
    void update(num delta) {
      final num diff = delta - _lastTimestamp;
  
      if (diff > GAME_SPEED) {
        _lastTimestamp = delta;
        clear();
        drawCell(_ctx, _food, "blue");
        _snake.update(_ctx, _keyboard);
        _checkForCollisions();
  
negative.code:   }
  
    void _checkForCollisions() {
      // check death conditions
      if (_snake.head.x <= -1 || _snake.head.x >= _rightEdgeX ||
        _snake.head.y <= -1 || _snake.head.y >= _bottomEdgeY ||
        _snake.checkForBodyCollision()) {
        init();
        return;
      }
  
      // check for collision with food
  