anchor.diff:      int score = 0;
       Text scoreText = new Text(this, "Score: $score", size: 20);
   
  +    Button leftButton = new Button(this, "leftButton", "")
  +        ..x = 10
  +        ..y = 510;
  +
  +    Button rightButton = new Button(this, "rightButton", '')
  +        ..x = 710
  +        ..y = 510;
  +
       Ship player = new Ship(this, "ship")
           ..center()
           ..x = 400
           ..y = 500
           ..vx = 300;
  +
  +    leftButton.onMouseDown.listen((_) {
  +      player.movingLeft = true;
  +    });
  +
  +    leftButton.onMouseUp.listen((_) {
  +      player.movingLeft = false;
  +    });
  +
  +    rightButton.onMouseDown.listen((_) {
  +      player.movingRight = true;
  +    });
  +
  +    rightButton.onMouseUp.listen((_) {
  +      player.movingRight = false;
  +    });
   
   
       Group<Sprite> bullets = new Group<Sprite>();
  
positive.code: 
  
      Sprite bullet;
      Timer bulletTimer = new Timer.periodic(new Duration(milliseconds: 450), (_) {
        if (bullets.any((item) => !item.alive)) {
          bullet = bullets.firstWhere((item) => !item.alive)
              ..x = player.x
  
negative.code:             scoreText.text = "Score: ${score+=10}";
            }
          });
          if (ufo.hitTestObject(player)) {
            game.stage.removeEventListeners("keyDown");
            bulletTimer.cancel();
            killteState();
  