test: Add initial integration tests

Refer to `docs/integration_tests.md` for
notes.

docs/integration_tests.md

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+# Integration Tests
+
+Integration tests in Flutter allow self-driven end-to-end
+tests of app code.
+
+The focus in this document are notes for using integration
+tests to capture performance metrics on physical devices.
+For more information on that topic see
+[Flutter cookbook on integration profiling][profiling-cookbook].
+
+For more background on integration testing in general
+see [Flutter docs on integration testing][flutter-docs].
+
+[flutter-docs]: https://docs.flutter.dev/testing/integration-tests
+[profiling-cookbook]: https://docs.flutter.dev/cookbook/testing/integration/profiling
+
+## Writing tests
+
+Writing an integration test involves two parts: test code
+that runs on a device and driver code that runs on the host.
+
+Integration test code is written in a similar style as
+widget test code, using a `testWidgets` function as well as
+a `WidgetTester` instance to arrange widgets and run
+interactions. A difference is the usage of
+`IntegrationTestWidgetsFlutterBinding` which provides a
+`traceAction` method used to record a Dart VM timeline.
+
+Driver code runs on the host and is useful to configure
+output of captured timeline data. There is a baseline driver
+at `integration_test/perf_driver.dart` that also creates a
+summary of timeline data to extract metrics such as widget
+build times and frame rendering performance.
+
+## Running tests
+
+The command to run an integration test:
+
+```
+$ flutter drive \
+  --driver=integration_test/perf_driver.dart \
+  --target=integration_test/unreadmarker_test.dart \
+  -d <device_id> \
+  --profile \
+  --no-dds
+```
+
+Obtain the `device_id` using `flutter devices`.
+
+From this command timeline data will be produced in
+`build/trace_output.timeline.json`. This is a raw data file
+that contains all manner of captured event timings.
+
+A more readily consumable file will also be produced in
+`build/trace_output.timeline_summary.json`. This contains
+widget build times and frame timings.

integration_test/perf_driver.dart

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+// This file is derived from BSD licensed example code in
+// Flutter documentation about integration tests profiling:
+//   https://docs.flutter.dev/cookbook/testing/integration/profiling#3-save-the-results-to-disk
+//
+// This integration driver configures output of timeline data
+// and a summary thereof from integration tests. See
+// [our docs on integration tests](../docs/integration_tests.md)
+// for background.
+
+import 'package:flutter_driver/flutter_driver.dart' as driver;
+import 'package:integration_test/integration_test_driver.dart';
+
+Future<void> main() {
+  return integrationDriver(
+    responseDataCallback: (data) async {
+      if (data != null) {
+        final timeline = driver.Timeline.fromJson(data['timeline']);
+        final summary = driver.TimelineSummary.summarize(timeline);
+        await summary.writeTimelineToFile(
+          'trace_output',
+          pretty: true,
+          includeSummary: true);
+      }
+    });
+}

integration_test/unreadmarker_test.dart

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+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:integration_test/integration_test.dart';
+import 'package:zulip/api/model/events.dart';
+import 'package:zulip/api/model/model.dart';
+import 'package:zulip/model/narrow.dart';
+import 'package:zulip/model/store.dart';
+import 'package:zulip/widgets/message_list.dart';
+import 'package:zulip/widgets/store.dart';
+
+import '../test/api/fake_api.dart';
+import '../test/example_data.dart' as eg;
+import '../test/model/binding.dart';
+import '../test/model/message_list_test.dart';
+
+void main() {
+  final binding = IntegrationTestWidgetsFlutterBinding.ensureInitialized();
+  TestZulipBinding.ensureInitialized();
+
+  late PerAccountStore store;
+  late FakeApiConnection connection;
+
+  Future<List<Message>> setupMessageListPage(WidgetTester tester, int messageCount) async {
+    addTearDown(testBinding.reset);
+    await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot());
+    store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
+    connection = store.connection as FakeApiConnection;
+
+    // prepare message list data
+    final messages = List.generate(messageCount, (index) {
+      return eg.streamMessage(id: index, flags: [MessageFlag.read]);
+    });
+    connection.prepare(json:
+      newestResult(foundOldest: true, messages: messages).toJson());
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: GlobalStoreWidget(
+          child: PerAccountStoreWidget(
+            accountId: eg.selfAccount.id,
+            child: const MessageListPage(narrow: AllMessagesNarrow())))));
+    await tester.pumpAndSettle();
+    return messages;
+  }
+
+  testWidgets('_UnreadMarker animation performance test', (tester) async {
+    final messages = await setupMessageListPage(tester, 500);
+
+    // short pause for buffer during video-capture
+    await tester.pump(const Duration(seconds: 2));
+
+    await binding.traceAction(() async {
+      store.handleEvent(eg.updateMessageFlagsRemoveEvent(
+        MessageFlag.read,
+        messages));
+      await tester.pumpAndSettle();
+      store.handleEvent(UpdateMessageFlagsAddEvent(
+        id: 1,
+        flag: MessageFlag.read,
+        messages: messages.map((e) => e.id).toList(),
+        all: false));
+      await tester.pumpAndSettle();
+    });
+  });
+}