Add flutter_web samples (#75)

web/gallery/lib/demo/material/persistent_bottom_sheet_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 PersistentBottomSheetDemo extends StatefulWidget {
+  static const String routeName = '/material/persistent-bottom-sheet';
+
+  @override
+  _PersistentBottomSheetDemoState createState() =>
+      _PersistentBottomSheetDemoState();
+}
+
+class _PersistentBottomSheetDemoState extends State<PersistentBottomSheetDemo> {
+  final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
+
+  VoidCallback _showBottomSheetCallback;
+
+  @override
+  void initState() {
+    super.initState();
+    _showBottomSheetCallback = _showBottomSheet;
+  }
+
+  void _showBottomSheet() {
+    setState(() {
+      // disable the button
+      _showBottomSheetCallback = null;
+    });
+    _scaffoldKey.currentState
+        .showBottomSheet<Null>((BuildContext context) {
+          final ThemeData themeData = Theme.of(context);
+          return Container(
+            decoration: BoxDecoration(
+                border:
+                    Border(top: BorderSide(color: themeData.disabledColor))),
+            child: Padding(
+              padding: const EdgeInsets.all(32.0),
+              child: Text(
+                'This is a Material persistent bottom sheet. Drag downwards to dismiss it.',
+                textAlign: TextAlign.center,
+                style: TextStyle(color: themeData.accentColor, fontSize: 24.0),
+              ),
+            ),
+          );
+        })
+        .closed
+        .whenComplete(() {
+          if (mounted) {
+            setState(() {
+              // re-enable the button
+              _showBottomSheetCallback = _showBottomSheet;
+            });
+          }
+        });
+  }
+
+  void _showMessage() {
+    showDialog<void>(
+      context: context,
+      builder: (BuildContext context) {
+        return AlertDialog(
+          content: const Text('You tapped the floating action button.'),
+          actions: <Widget>[
+            FlatButton(
+                onPressed: () {
+                  Navigator.pop(context);
+                },
+                child: const Text('OK'))
+          ],
+        );
+      },
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+        key: _scaffoldKey,
+        appBar: AppBar(
+          title: const Text('Persistent bottom sheet'),
+          actions: <Widget>[
+            MaterialDemoDocumentationButton(
+                PersistentBottomSheetDemo.routeName),
+          ],
+        ),
+        floatingActionButton: FloatingActionButton(
+          onPressed: _showMessage,
+          backgroundColor: Colors.redAccent,
+          child: const Icon(
+            Icons.add,
+            semanticLabel: 'Add',
+          ),
+        ),
+        body: Center(
+            child: RaisedButton(
+                onPressed: _showBottomSheetCallback,
+                child: const Text('SHOW BOTTOM SHEET'))));
+  }
+}