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/api/api.dart

@@ -0,0 +1,45 @@
+// 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.
+
+/// Manipulates app data,
+abstract class DashboardApi {
+  ItemApi get items;
+  EntryApi get entries;
+}
+
+/// Manipulates [Item] data.
+abstract class ItemApi {
+  Future<Item> delete(String id);
+  Future<Item> get(String id);
+  Future<Item> insert(Item item);
+  Future<List<Item>> list();
+  Future<Item> update(Item item, String id);
+  Stream<List<Item>> allItemsStream();
+}
+
+/// Something being tracked.
+class Item {
+  final String name;
+  String id;
+
+  Item(this.name);
+}
+
+/// Manipulates [Entry] data.
+abstract class EntryApi {
+  Future<Entry> delete(String itemId, String id);
+  Future<Entry> insert(String itemId, Entry entry);
+  Future<List<Entry>> list(String itemId);
+  Future<Entry> update(String itemId, String id, Entry entry);
+  Stream<List<Entry>> allEntriesStream(String itemId);
+}
+
+/// A number tracked at a point in time.
+class Entry {
+  final int value;
+  final DateTime time;
+  String id;
+
+  Entry(this.value, this.time);
+}