Initial date planner app (#2390)

Initial **experimental** date planner iOS sample.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

experimental/date_planner/lib/event_list.dart

@@ -0,0 +1,99 @@
+// Copyright 2024 The Flutter 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/cupertino.dart';
+import 'package:provider/provider.dart';
+
+import 'event.dart';
+import 'event_data.dart';
+import 'event_editor.dart';
+import 'event_row.dart';
+
+class EventList extends StatelessWidget {
+  const EventList({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return Consumer<EventData>(
+      builder: (BuildContext context, EventData events, Widget? child) {
+        return CupertinoPageScaffold(
+          // TODO(mit-mit): Avoid having to pass nav bar manually.
+          //
+          // Would like to pass nav bar and body to CupertinoPageScaffold
+          // directly, similar to the Material Scaffold's `appBar` and `body`
+          // args.
+          // https://github.com/flutter/flutter/issues/149625
+          child: CustomScrollView(
+            slivers: <Widget>[
+              CupertinoSliverNavigationBar(
+                largeTitle: const Text('Date Planner'),
+                trailing: CupertinoButton(
+                  padding: EdgeInsets.zero,
+                  child: const Icon(CupertinoIcons.plus),
+                  onPressed: () async {
+                    // Issue: Should go to a sheet, not a a full-screen page.
+                    // Blocked on https://github.com/flutter/flutter/issues/42560.
+                    Event? newEvent = await Navigator.of(context).push(
+                      CupertinoPageRoute<Event>(
+                        builder: (_) => EventEditor(
+                          event: Event(title: 'New event'),
+                          isNew: true,
+                        ),
+                      ),
+                    );
+
+                    if (newEvent != null) {
+                      events.add(newEvent);
+                    }
+                  },
+                ),
+              ),
+              SliverList.list(
+                children: [
+                  for (Period p in Period.values)
+                    CupertinoListSection(
+                      header: Text(
+                        p.name.toUpperCase(),
+                        style: const TextStyle(
+                          fontWeight: FontWeight.bold,
+                          fontSize: 16,
+                        ),
+                      ),
+                      children: [
+                        for (Event e in events.sorted(p))
+                          // TODO: Support swipe action for deleting.
+                          // Should probably use Dismissable?
+                          // https://api.flutter.dev/flutter/widgets/Dismissible-class.html
+                          EventRow(
+                            event: e,
+                            onTap: () async {
+                              Event? updatedEvent =
+                                  await Navigator.of(context).push(
+                                CupertinoPageRoute<Event>(
+                                  builder: (_) => EventEditor(
+                                    event: e.copy(),
+                                    isNew: false,
+                                  ),
+                                ),
+                              );
+                              if (updatedEvent == null) {
+                                // The editor passes back null when it deleted
+                                // the element.
+                                events.delete(e);
+                              } else {
+                                events.update(e, updatedEvent);
+                              }
+                            },
+                          )
+                      ],
+                    ),
+                ],
+              ),
+            ],
+          ),
+        );
+      },
+    );
+  }
+}