Add provider_shopper (#87)

This is the code for the sample at https://flutter.dev/docs/development/data-and-backend/state-mgmt/simple.

provider_shopper/lib/main.dart

@@ -0,0 +1,44 @@
+// Copyright 2019 The Flutter team. 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 'package:provider_shopper/common/theme.dart';
+import 'package:provider_shopper/models/cart.dart';
+import 'package:provider_shopper/models/catalog.dart';
+import 'package:provider_shopper/screens/cart.dart';
+import 'package:provider_shopper/screens/catalog.dart';
+
+void main() {
+  runApp(MyApp());
+}
+
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    // Using MultiProvider is convenient when providing multiple objects.
+    return MultiProvider(
+      providers: [
+        // In this sample app, CatalogModel never changes, so a simple Provider
+        // is sufficient.
+        Provider(builder: (context) => CatalogModel()),
+        // CartModel is implemented as a ChangeNotifier, which calls for the use
+        // of ChangeNotifierProvider. Moreover, CartModel depends
+        // on CatalogModel, so a ProxyProvider is needed.
+        ChangeNotifierProxyProvider<CatalogModel, CartModel>(
+            builder: (context, catalog, previousCart) =>
+                CartModel(catalog, previousCart)),
+      ],
+      child: MaterialApp(
+        title: 'Provider Demo',
+        theme: appTheme,
+        initialRoute: '/',
+        routes: {
+          '/': (context) => MyCatalog(),
+          '/cart': (context) => MyCart(),
+        },
+      ),
+    );
+  }
+}