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/api/page.dart

@@ -0,0 +1,24 @@
+// 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:meta/meta.dart';
+
+import 'item.dart';
+
+const int itemsPerPage = 20;
+
+class ItemPage {
+  final List<Item> items;
+
+  final int startingIndex;
+
+  final bool hasNext;
+
+  ItemPage({
+    @required this.items,
+    @required this.startingIndex,
+    @required this.hasNext,
+  });
+}