nav [nfc]: Make navigation more cleanly testable, with WidgetRoute

As discussed when we added this first test of this kind:
  #249 (comment)

Author: gnprice

lib/widgets/message_list.dart

@@ -20,9 +20,8 @@ class MessageListPage extends StatefulWidget {
   const MessageListPage({super.key, required this.narrow});
 
   static Route<void> buildRoute({required BuildContext context, required Narrow narrow}) {
-    return MaterialAccountPageRoute(context: context,
-      settings: RouteSettings(name: 'message_list', arguments: narrow), // for testing
-      builder: (context) => MessageListPage(narrow: narrow));
+    return MaterialAccountWidgetRoute(context: context,
+      page: MessageListPage(narrow: narrow));
   }
 
   /// A [ComposeBoxController], if this [MessageListPage] offers a compose box.

lib/widgets/page.dart

@@ -3,6 +3,30 @@ import 'package:flutter/material.dart';
 
 import 'store.dart';
 
+/// A page route that always builds the same widget.
+///
+/// This is useful for making the route more transparent for a test to inspect.
+abstract class WidgetRoute<T> extends PageRoute<T> {
+  /// The widget that this page route always builds.
+  Widget get page;
+}
+
+/// A [MaterialPageRoute] that always builds the same widget.
+///
+/// This is useful for making the route more transparent for a test to inspect.
+class MaterialWidgetRoute<T> extends MaterialPageRoute<T> implements WidgetRoute<T> {
+  MaterialWidgetRoute({
+    required this.page,
+    super.settings,
+    super.maintainState,
+    super.fullscreenDialog,
+    super.allowSnapshotting,
+  }) : super(builder: (context) => page);
+
+  @override
+  final Widget page;
+}
+
 mixin AccountPageRouteMixin<T> on PageRoute<T> {
   int get accountId;
 
@@ -28,6 +52,23 @@ class MaterialAccountPageRoute<T> extends MaterialPageRoute<T> with AccountPageR
   final int accountId;
 }
 
+/// A [MaterialAccountPageRoute] that always builds the same widget.
+///
+/// This is useful for making the route more transparent for a test to inspect.
+class MaterialAccountWidgetRoute<T> extends MaterialAccountPageRoute<T> implements WidgetRoute<T> {
+  MaterialAccountWidgetRoute({
+    required super.context,
+    required this.page,
+    super.settings,
+    super.maintainState,
+    super.fullscreenDialog,
+    super.allowSnapshotting,
+  }) : super(builder: (context) => page);
+
+  @override
+  final Widget page;
+}
+
 class AccountPageRouteBuilder<T> extends PageRouteBuilder<T> with AccountPageRouteMixin<T> {
   AccountPageRouteBuilder({
     required BuildContext context,

test/widgets/message_list_checks.dart

@@ -0,0 +1,7 @@
+import 'package:checks/checks.dart';
+import 'package:zulip/model/narrow.dart';
+import 'package:zulip/widgets/message_list.dart';
+
+extension MessageListPageChecks on Subject<MessageListPage> {
+  Subject<Narrow> get narrow => has((x) => x.narrow, 'narrow');
+}

test/widgets/page_checks.dart

@@ -0,0 +1,7 @@
+import 'package:checks/checks.dart';
+import 'package:flutter/widgets.dart';
+import 'package:zulip/widgets/page.dart';
+
+extension WidgetRouteChecks on Subject<WidgetRoute> {
+  Subject<Widget> get page => has((x) => x.page, 'page');
+}

test/widgets/recent_dm_conversations_test.dart

@@ -7,15 +7,18 @@ import 'package:zulip/api/model/model.dart';
 import 'package:zulip/model/narrow.dart';
 import 'package:zulip/widgets/content.dart';
 import 'package:zulip/widgets/icons.dart';
+import 'package:zulip/widgets/message_list.dart';
+import 'package:zulip/widgets/page.dart';
 import 'package:zulip/widgets/recent_dm_conversations.dart';
 import 'package:zulip/widgets/store.dart';
 
 import '../example_data.dart' as eg;
-import '../flutter_checks.dart';
 import '../model/binding.dart';
 import '../model/test_store.dart';
 import '../test_navigation.dart';
 import 'content_checks.dart';
+import 'message_list_checks.dart';
+import 'page_checks.dart';
 
 Future<void> setupPage(WidgetTester tester, {
   required List<DmMessage> dmMessages,
@@ -275,9 +278,9 @@ void main() {
         await tester.tap(find.byType(RecentDmConversationsItem));
         // no `tester.pump`, to avoid having to mock API response for [MessageListPage]
 
-        check(pushedRoutes).last.settings
-          ..name.equals('message_list')
-          ..arguments.equals(expectedNarrow);
+        check(pushedRoutes).last.isA<WidgetRoute>().page
+          .isA<MessageListPage>()
+          .narrow.equals(expectedNarrow);
       }
 
       testWidgets('1:1', (WidgetTester tester) async {