Add flutter_web samples (#75)

web/gallery/lib/demo/animation_demo.dart

@@ -0,0 +1,16 @@
+// Copyright 2018 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 'package:flutter_web/material.dart';
+
+import 'animation/home.dart';
+
+class AnimationDemo extends StatelessWidget {
+  const AnimationDemo({Key key}) : super(key: key);
+
+  static const String routeName = '/animation';
+
+  @override
+  Widget build(BuildContext context) => const AnimationDemoHome();
+}