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/test/smoke_test.dart

@@ -0,0 +1,40 @@
+// 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:flutter_test/flutter_test.dart';
+
+import 'package:infinitelist/main.dart';
+
+void main() {
+  testWidgets('Infinite list smoke test', (tester) async {
+    await tester.pumpWidget(MyApp());
+
+    const loadingDuration = Duration(milliseconds: 500);
+
+    // At first, the catalog shows only "..." (loading items).
+    expect(find.text('...'), findsWidgets);
+    expect(find.text('Color #1'), findsNothing);
+
+    await tester.pump(loadingDuration);
+
+    // After the app has had a chance to load items, it should no longer
+    // show "...".
+    expect(find.text('...'), findsNothing);
+    expect(find.text('Color #1'), findsOneWidget);
+
+    // Flinging up quickly (i.e. scrolling down).
+    await tester.fling(find.byType(ListView), Offset(0, -2000), 5000);
+
+    // As we scroll down, we should see more items loading.
+    expect(find.text('...'), findsWidgets);
+    // The item at the top should no longer be in view.
+    expect(find.text('Color #1'), findsNothing);
+
+    // This last part is just giving the app time to complete its "fetch"
+    // of new items. Otherwise, the test fails because of outstanding
+    // asynchronous callbacks.
+    await tester.pump(loadingDuration);
+  });
+}