Add flutter_web samples (#75)

web/slide_puzzle/lib/src/app_state.dart

@@ -0,0 +1,31 @@
+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'core/puzzle_animator.dart';
+import 'flutter.dart';
+import 'shared_theme.dart';
+
+abstract class AppState {
+  TabController get tabController;
+
+  PuzzleProxy get puzzle;
+
+  bool get autoPlay;
+
+  void setAutoPlay(bool newValue);
+
+  AnimationNotifier get animationNotifier;
+
+  Iterable<SharedTheme> get themeData;
+
+  SharedTheme get currentTheme;
+
+  set currentTheme(SharedTheme theme);
+}
+
+abstract class AnimationNotifier implements Listenable {
+  void animate();
+
+  void dispose();
+}