Add flutter_web samples (#75)

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

@@ -0,0 +1,92 @@
+// 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 'dart:async';
+
+import 'package:flutter_web/material.dart';
+
+import '../../gallery/demo.dart';
+
+enum IndicatorType { overscroll, refresh }
+
+class OverscrollDemo extends StatefulWidget {
+  const OverscrollDemo({Key key}) : super(key: key);
+
+  static const String routeName = '/material/overscroll';
+
+  @override
+  OverscrollDemoState createState() => OverscrollDemoState();
+}
+
+class OverscrollDemoState extends State<OverscrollDemo> {
+  final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
+  final GlobalKey<RefreshIndicatorState> _refreshIndicatorKey =
+      GlobalKey<RefreshIndicatorState>();
+  static final List<String> _items = <String>[
+    'A',
+    'B',
+    'C',
+    'D',
+    'E',
+    'F',
+    'G',
+    'H',
+    'I',
+    'J',
+    'K',
+    'L',
+    'M',
+    'N'
+  ];
+
+  Future<void> _handleRefresh() {
+    final Completer<void> completer = Completer<void>();
+    Timer(const Duration(seconds: 3), () {
+      completer.complete();
+    });
+    return completer.future.then<void>((_) {
+      _scaffoldKey.currentState?.showSnackBar(SnackBar(
+          content: const Text('Refresh complete'),
+          action: SnackBarAction(
+              label: 'RETRY',
+              onPressed: () {
+                _refreshIndicatorKey.currentState.show();
+              })));
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      key: _scaffoldKey,
+      appBar: AppBar(title: const Text('Pull to refresh'), actions: <Widget>[
+        MaterialDemoDocumentationButton(OverscrollDemo.routeName),
+        IconButton(
+            icon: const Icon(Icons.refresh),
+            tooltip: 'Refresh',
+            onPressed: () {
+              _refreshIndicatorKey.currentState.show();
+            }),
+      ]),
+      body: RefreshIndicator(
+        key: _refreshIndicatorKey,
+        onRefresh: _handleRefresh,
+        child: ListView.builder(
+          padding: kMaterialListPadding,
+          itemCount: _items.length,
+          itemBuilder: (BuildContext context, int index) {
+            final String item = _items[index];
+            return ListTile(
+              isThreeLine: true,
+              leading: CircleAvatar(child: Text(item)),
+              title: Text('This item represents $item.'),
+              subtitle: const Text(
+                  'Even more additional list item information appears on line three.'),
+            );
+          },
+        ),
+      ),
+    );
+  }
+}