Add flutter_web samples (#75)

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

@@ -0,0 +1,83 @@
+// 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';
+
+const String _text1 =
+    'Snackbars provide lightweight feedback about an operation by '
+    'showing a brief message at the bottom of the screen. Snackbars '
+    'can contain an action.';
+
+const String _text2 =
+    'Snackbars should contain a single line of text directly related '
+    'to the operation performed. They cannot contain icons.';
+
+const String _text3 =
+    'By default snackbars automatically disappear after a few seconds ';
+
+class SnackBarDemo extends StatefulWidget {
+  const SnackBarDemo({Key key}) : super(key: key);
+
+  static const String routeName = '/material/snack-bar';
+
+  @override
+  _SnackBarDemoState createState() => _SnackBarDemoState();
+}
+
+class _SnackBarDemoState extends State<SnackBarDemo> {
+  int _snackBarIndex = 1;
+
+  Widget buildBody(BuildContext context) {
+    return SafeArea(
+      top: false,
+      bottom: false,
+      child: ListView(
+          padding: const EdgeInsets.all(24.0),
+          children: <Widget>[
+            const Text(_text1),
+            const Text(_text2),
+            Center(
+              child: Row(children: <Widget>[
+                RaisedButton(
+                    child: const Text('SHOW A SNACKBAR'),
+                    onPressed: () {
+                      final int thisSnackBarIndex = _snackBarIndex++;
+                      Scaffold.of(context).showSnackBar(SnackBar(
+                        content: Text('This is snackbar #$thisSnackBarIndex.'),
+                        action: SnackBarAction(
+                            label: 'ACTION',
+                            onPressed: () {
+                              Scaffold.of(context).showSnackBar(SnackBar(
+                                  content: Text(
+                                      'You pressed snackbar $thisSnackBarIndex\'s action.')));
+                            }),
+                      ));
+                    }),
+              ]),
+            ),
+            const Text(_text3),
+          ].map<Widget>((Widget child) {
+            return Container(
+                margin: const EdgeInsets.symmetric(vertical: 12.0),
+                child: child);
+          }).toList()),
+    );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+        appBar: AppBar(
+          title: const Text('Snackbar'),
+          actions: <Widget>[
+            MaterialDemoDocumentationButton(SnackBarDemo.routeName)
+          ],
+        ),
+        body: Builder(
+            // Create an inner BuildContext so that the snackBar onPressed methods
+            // can refer to the Scaffold with Scaffold.of().
+            builder: buildBody));
+  }
+}