add web_dashboard API (#333)

* add web_dashboard sample

* add docs

* address code review comments

* restructure web_dashboard

* set up provider and mock service

* add copyright headers, use relative imports

* add API class, add tests

* fmt

* rename services -> API, remove data library

* use new API in app

* add stream to items api

* convert from StreamBuilder to StreamProvider

* add subscription to Entry API

* rename API classes

* address comments

* Update README.md

* update README

* remove routing_demo

experimental/web_dashboard/lib/src/app.dart

@@ -0,0 +1,25 @@
+// Copyright 2020, the Flutter project authors. Please see the AUTHORS file
+// for details. 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/material.dart';
+import 'package:provider/provider.dart';
+
+import 'pages/home.dart';
+import 'api/api.dart';
+import 'api/mock.dart';
+
+/// An app that shows a responsive dashboard.
+class DashboardApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MultiProvider(
+      providers: [
+        Provider<DashboardApi>(create: (_) => MockDashboardApi()),
+      ],
+      child: MaterialApp(
+        home: HomePage(),
+      ),
+    );
+  }
+}