game_route.dart

class _GameRouteState extends State<GameRoute> implements QuizEventListener {
  
  ...

  // TODO: Add _interstitialAd
  InterstitialAd _interstitialAd;

  // TODO: Add _isInterstitialAdReady
  bool _isInterstitialAdReady;

  ...

  // TODO: Implement _loadInterstitialAd()
  void _loadInterstitialAd() {
    _interstitialAd.load();
  }

  // TODO: Implement _onInterstitialAdEvent()
  void _onInterstitialAdEvent(MobileAdEvent event) {
    switch (event) {
      case MobileAdEvent.loaded:
        _isInterstitialAdReady = true;
        break;
      case MobileAdEvent.failedToLoad:
        _isInterstitialAdReady = false;
        print('Failed to load an interstitial ad');
        break;
      case MobileAdEvent.closed:
        _moveToHome();
        break;
      default:
      // do nothing
    }
  }

  ...
}