Add flutter_web samples (#75)

web/gallery/lib/demo/material/stack_demo.dart

@@ -0,0 +1,23 @@
+// 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';
+
+class StackDemo extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return DecoratedBox(
+      decoration: BoxDecoration(
+        border: Border.all(
+          color: Colors.greenAccent,
+          width: 1.0,
+        ),
+      ),
+      child: Stack(children: [
+        Text('A'),
+        Text('B'),
+      ]),
+    );
+  }
+}