Add the infinite_list sample (#440)

This PR adds a Flutter sample app that shows an implementation of the "infinite list" UX pattern. That is, a list is shown to the user as if it was continuous although it is internally paginated. This is a common feature of mobile apps, from shopping catalogs through search engines to social media clients.

infinite_list/lib/main.dart

@@ -0,0 +1,66 @@
+// Copyright 2020 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/material.dart';
+import 'package:provider/provider.dart';
+
+import 'src/catalog.dart';
+import 'src/item_tile.dart';
+
+void main() {
+  runApp(MyApp());
+}
+
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return ChangeNotifierProvider<Catalog>(
+      create: (context) => Catalog(),
+      child: MaterialApp(
+        title: 'Infinite List Sample',
+        home: MyHomePage(),
+      ),
+    );
+  }
+}
+
+class MyHomePage extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: Text('Infinite List Sample'),
+      ),
+      body: Selector<Catalog, int>(
+        // Selector is a widget from package:provider. It allows us to listen
+        // to only one aspect of a provided value. In this case, we are only
+        // listening to the catalog's `itemCount`, because that's all we need
+        // at this level.
+        selector: (context, catalog) => catalog.itemCount,
+        builder: (context, itemCount, child) => ListView.builder(
+          // When `itemCount` is null, `ListView` assumes an infinite list.
+          // Once we provide a value, it will stop the scrolling beyond
+          // the last element.
+          itemCount: itemCount,
+          padding: const EdgeInsets.symmetric(vertical: 18),
+          itemBuilder: (context, index) {
+            // Every item of the `ListView` is individually listening
+            // to the catalog.
+            var catalog = Provider.of<Catalog>(context);
+
+            // Catalog provides a single synchronous method for getting
+            // the current data.
+            var item = catalog.getByIndex(index);
+
+            if (item.isLoading) {
+              return LoadingItemTile();
+            }
+
+            return ItemTile(item: item);
+          },
+        ),
+      ),
+    );
+  }
+}