Add flutter_web samples (#75)

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

@@ -0,0 +1,59 @@
+// 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 _introText =
+    'Tooltips are short identifying messages that briefly appear in response to '
+    'a long press. Tooltip messages are also used by services that make Flutter '
+    'apps accessible, like screen readers.';
+
+class TooltipDemo extends StatelessWidget {
+  static const String routeName = '/material/tooltips';
+
+  @override
+  Widget build(BuildContext context) {
+    final ThemeData theme = Theme.of(context);
+    return Scaffold(
+        appBar: AppBar(
+          title: const Text('Tooltips'),
+          actions: <Widget>[MaterialDemoDocumentationButton(routeName)],
+        ),
+        body: Builder(builder: (BuildContext context) {
+          return SafeArea(
+            top: false,
+            bottom: false,
+            child: ListView(
+                children: <Widget>[
+              Text(_introText, style: theme.textTheme.subhead),
+              Row(children: <Widget>[
+                Text('Long press the ', style: theme.textTheme.subhead),
+                Tooltip(
+                    message: 'call icon',
+                    child: Icon(Icons.call,
+                        size: 18.0, color: theme.iconTheme.color)),
+                Text(' icon.', style: theme.textTheme.subhead)
+              ]),
+              Center(
+                  child: IconButton(
+                      iconSize: 48.0,
+                      icon: const Icon(Icons.call),
+                      color: theme.iconTheme.color,
+                      tooltip: 'Place a phone call',
+                      onPressed: () {
+                        Scaffold.of(context).showSnackBar(const SnackBar(
+                            content: Text('That was an ordinary tap.')));
+                      }))
+            ].map<Widget>((Widget widget) {
+              return Padding(
+                  padding:
+                      const EdgeInsets.only(top: 16.0, left: 16.0, right: 16.0),
+                  child: widget);
+            }).toList()),
+          );
+        }));
+  }
+}