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/screens/cart.dart

@@ -0,0 +1,83 @@
+// 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/models/cart.dart';
+
+class MyCart extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Cart', style: Theme.of(context).textTheme.display4),
+        backgroundColor: Colors.white,
+      ),
+      body: Container(
+        color: Colors.yellow,
+        child: Column(
+          children: [
+            Expanded(
+              child: Padding(
+                padding: const EdgeInsets.all(32),
+                child: _CartList(),
+              ),
+            ),
+            Divider(height: 4, color: Colors.black),
+            _CartTotal()
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+class _CartList extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    var itemNameStyle = Theme.of(context).textTheme.title;
+    var cart = Provider.of<CartModel>(context);
+
+    return ListView.builder(
+      itemCount: cart.items.length,
+      itemBuilder: (context, index) => ListTile(
+        leading: Icon(Icons.done),
+        title: Text(
+          cart.items[index].name,
+          style: itemNameStyle,
+        ),
+      ),
+    );
+  }
+}
+
+class _CartTotal extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    var hugeStyle = Theme.of(context).textTheme.display4.copyWith(fontSize: 48);
+
+    return SizedBox(
+      height: 200,
+      child: Center(
+        child: Row(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            Consumer<CartModel>(
+                builder: (context, cart, child) =>
+                    Text('\$${cart.totalPrice}', style: hugeStyle)),
+            SizedBox(width: 24),
+            FlatButton(
+              onPressed: () {
+                Scaffold.of(context).showSnackBar(
+                    SnackBar(content: Text('Buying not supported yet.')));
+              },
+              color: Colors.white,
+              child: Text('BUY'),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}