Add flutter_web samples (#75)

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

@@ -0,0 +1,69 @@
+// 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 ElevationDemo extends StatefulWidget {
+  static const String routeName = '/material/elevation';
+
+  @override
+  State<StatefulWidget> createState() => _ElevationDemoState();
+}
+
+class _ElevationDemoState extends State<ElevationDemo> {
+  bool _showElevation = true;
+
+  List<Widget> buildCards() {
+    const List<double> elevations = <double>[
+      0.0,
+      1.0,
+      2.0,
+      3.0,
+      4.0,
+      5.0,
+      8.0,
+      16.0,
+      24.0,
+    ];
+
+    return elevations.map<Widget>((double elevation) {
+      return Center(
+        child: Card(
+          margin: const EdgeInsets.all(20.0),
+          elevation: _showElevation ? elevation : 0.0,
+          child: SizedBox(
+            height: 100.0,
+            width: 100.0,
+            child: Center(
+              child: Text('${elevation.toStringAsFixed(0)} pt'),
+            ),
+          ),
+        ),
+      );
+    }).toList();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text('Elevation'),
+        actions: <Widget>[
+          MaterialDemoDocumentationButton(ElevationDemo.routeName),
+          IconButton(
+            icon: const Icon(Icons.sentiment_very_satisfied),
+            onPressed: () {
+              setState(() => _showElevation = !_showElevation);
+            },
+          )
+        ],
+      ),
+      body: ListView(
+        children: buildCards(),
+      ),
+    );
+  }
+}