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/src/item_tile.dart

@@ -0,0 +1,53 @@
+// 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 'api/item.dart';
+
+/// This is the widget responsible for building the item in the list,
+/// once we have the actual data [item].
+class ItemTile extends StatelessWidget {
+  final Item item;
+
+  ItemTile({@required this.item, Key key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Padding(
+      padding: const EdgeInsets.all(8.0),
+      child: ListTile(
+        leading: AspectRatio(
+          aspectRatio: 1,
+          child: Container(
+            color: item.color,
+          ),
+        ),
+        title: Text(item.name, style: Theme.of(context).textTheme.headline6),
+        trailing: Text('\$ ${(item.price / 100).toStringAsFixed(2)}'),
+      ),
+    );
+  }
+}
+
+/// This is the widget responsible for building the "still loading" item
+/// in the list (represented with "..." and a crossed square).
+class LoadingItemTile extends StatelessWidget {
+  const LoadingItemTile({Key key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Padding(
+      padding: const EdgeInsets.all(8.0),
+      child: ListTile(
+        leading: AspectRatio(
+          aspectRatio: 1,
+          child: Placeholder(),
+        ),
+        title: Text('...', style: Theme.of(context).textTheme.headline6),
+        trailing: Text('\$ ...'),
+      ),
+    );
+  }
+}