Add flutter_web samples (#75)

web/gallery/lib/main_houdini.dart

@@ -0,0 +1,12 @@
+// 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_ui/ui.dart' as ui;
+
+import 'main.dart' as app;
+
+void main() {
+  ui.persistedPictureFactory = ui.houdiniPictureFactory;
+  app.main();
+}