Add game_template (#1180)

Adds a template / sample for games built in Flutter, with all the bells and whistles, like ads, in-app purchases, audio, main menu, settings, and so on.

Co-authored-by: Parker Lougheed
Co-authored-by: Shams Zakhour

game_template/lib/src/ads/preloaded_banner_ad.dart

@@ -0,0 +1,71 @@
+// Copyright 2022, the Flutter project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:io';
+
+import 'package:google_mobile_ads/google_mobile_ads.dart';
+import 'package:logging/logging.dart';
+
+class PreloadedBannerAd {
+  static final _log = Logger('PreloadedBannerAd');
+
+  /// Something like [AdSize.mediumRectangle].
+  final AdSize size;
+
+  final AdRequest _adRequest;
+
+  BannerAd? _bannerAd;
+
+  final String adUnitId;
+
+  final _adCompleter = Completer<BannerAd>();
+
+  PreloadedBannerAd({
+    required this.size,
+    required this.adUnitId,
+    AdRequest? adRequest,
+  }) : _adRequest = adRequest ?? const AdRequest();
+
+  Future<BannerAd> get ready => _adCompleter.future;
+
+  Future<void> load() {
+    assert(Platform.isAndroid || Platform.isIOS,
+        'AdMob currently does not support ${Platform.operatingSystem}');
+
+    _bannerAd = BannerAd(
+      // This is a test ad unit ID from
+      // https://developers.google.com/admob/android/test-ads. When ready,
+      // you replace this with your own, production ad unit ID,
+      // created in https://apps.admob.com/.
+      adUnitId: adUnitId,
+      size: size,
+      request: _adRequest,
+      listener: BannerAdListener(
+        onAdLoaded: (ad) {
+          _log.info(() => 'Ad loaded: ${_bannerAd.hashCode}');
+          _adCompleter.complete(_bannerAd);
+        },
+        onAdFailedToLoad: (ad, error) {
+          _log.warning('Banner failedToLoad: $error');
+          _adCompleter.completeError(error);
+          ad.dispose();
+        },
+        onAdImpression: (ad) {
+          _log.info('Ad impression registered');
+        },
+        onAdClicked: (ad) {
+          _log.info('Ad click registered');
+        },
+      ),
+    );
+
+    return _bannerAd!.load();
+  }
+
+  void dispose() {
+    _log.info('preloaded banner ad being disposed');
+    _bannerAd?.dispose();
+  }
+}