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/fetch.dart

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+// 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 'item.dart';
+import 'page.dart';
+
+const catalogLength = 200;
+
+/// This function emulates a REST API call. You can imagine replacing its
+/// contents with an actual network call, keeping the signature the same.
+///
+/// It will fetch a page of items from [startingIndex].
+Future<ItemPage> fetchPage(int startingIndex) async {
+  // We're emulating the delay inherent to making a network call.
+  await Future<void>.delayed(const Duration(milliseconds: 500));
+
+  // If the [startingIndex] is beyond the bounds of the catalog, an
+  // empty page will be returned.
+  if (startingIndex > catalogLength) {
+    return ItemPage(
+      items: [],
+      startingIndex: startingIndex,
+      hasNext: false,
+    );
+  }
+
+  // The page of items is generated here.
+  return ItemPage(
+    items: List.generate(
+        itemsPerPage,
+        (index) => Item(
+              color: Colors.primaries[index % Colors.primaries.length],
+              name: 'Color #${startingIndex + index}',
+              price: 50 + (index * 42) % 200,
+            )),
+    startingIndex: startingIndex,
+    // Returns `false` if we've reached the [catalogLength].
+    hasNext: startingIndex + itemsPerPage < catalogLength,
+  );
+}

infinite_list/lib/src/api/item.dart

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+// 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';
+
+class Item {
+  final Color color;
+
+  final int price;
+
+  final String name;
+
+  Item({
+    @required this.color,
+    @required this.name,
+    @required this.price,
+  });
+
+  Item.loading() : this(color: Colors.grey, name: '...', price: 0);
+
+  bool get isLoading => name == '...';
+}

infinite_list/lib/src/api/page.dart

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+// 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,
+  });
+}