Add flutter_web samples (#75)

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

@@ -0,0 +1,103 @@
+// 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 '../../gallery/demo.dart';
+
+class ButtonsDemo extends StatelessWidget {
+  static const String routeName = '/material/buttons';
+  final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
+
+  @override
+  Widget build(BuildContext context) {
+    IconData _backIcon() {
+      switch (Theme.of(context).platform) {
+        case TargetPlatform.android:
+        case TargetPlatform.fuchsia:
+          return Icons.arrow_back;
+        case TargetPlatform.iOS:
+          return Icons.arrow_back_ios;
+      }
+      assert(false);
+      return null;
+    }
+
+    return Scaffold(
+      key: _scaffoldKey,
+      appBar: AppBar(
+        leading: IconButton(
+          icon: Icon(_backIcon()),
+          alignment: Alignment.centerLeft,
+          tooltip: 'Back',
+          onPressed: () {
+            Navigator.pop(context);
+          },
+        ),
+        title: const Text('Material buttons'),
+        actions: <Widget>[
+          MaterialDemoDocumentationButton(ButtonsDemo.routeName)
+        ],
+      ),
+      body: Center(
+        child: _buildButtons(),
+      ),
+    );
+  }
+
+  Widget _buildButtons() {
+    return Column(
+      children: [
+        pad(MaterialButton(
+          onPressed: () {
+            print('MaterialButton pressed');
+          },
+          elevation: 3.0,
+          child: Text('MaterialButton'),
+        )),
+        pad(FlatButton(
+          onPressed: () {
+            print('FlatButton pressed');
+          },
+          child: Text('FlatButton'),
+        )),
+        pad(RaisedButton(
+          onPressed: () {},
+          elevation: 0.0,
+          child: Text('RaisedButton 0.0'),
+        )),
+        pad(RaisedButton(
+          onPressed: () {},
+          elevation: 1.0,
+          child: Text('RaisedButton 1.0'),
+        )),
+        pad(RaisedButton(
+          onPressed: () {},
+          elevation: 2.0,
+          child: Text('RaisedButton 2.0'),
+        )),
+        pad(RaisedButton(
+          onPressed: () {},
+          elevation: 3.0,
+          child: Text('RaisedButton 3.0'),
+        )),
+        pad(RaisedButton(
+          onPressed: () {},
+          elevation: 4.0,
+          child: Text('RaisedButton 4.0'),
+        )),
+        pad(RaisedButton(
+          onPressed: () {},
+          elevation: 8.0,
+          child: Text('RaisedButton 8.0'),
+        )),
+      ],
+    );
+  }
+}
+
+Padding pad(Widget widget) => Padding(
+      padding: EdgeInsets.all(10.0),
+      child: widget,
+    );