Add flutter_web samples (#75)

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

@@ -0,0 +1,38 @@
+// 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 ModalBottomSheetDemo extends StatelessWidget {
+  static const String routeName = '/material/modal-bottom-sheet';
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+        appBar: AppBar(
+          title: const Text('Modal bottom sheet'),
+          actions: <Widget>[MaterialDemoDocumentationButton(routeName)],
+        ),
+        body: Center(
+            child: RaisedButton(
+                child: const Text('SHOW BOTTOM SHEET'),
+                onPressed: () {
+                  showModalBottomSheet<void>(
+                      context: context,
+                      builder: (BuildContext context) {
+                        return Container(
+                            child: Padding(
+                                padding: const EdgeInsets.all(32.0),
+                                child: Text(
+                                    'This is the modal bottom sheet. Tap anywhere to dismiss.',
+                                    textAlign: TextAlign.center,
+                                    style: TextStyle(
+                                        color: Theme.of(context).accentColor,
+                                        fontSize: 24.0))));
+                      });
+                })));
+  }
+}