msglist [nfc]: Set route settings on message-list page route, for testing

This way, the Route object itself is distinguishable from other
Route objects. This will come in handy in widget tests for the
upcoming recent-DMs screen (zulip#119), where we'll want to assert that
tapping on a conversation brings up the appropriate message list.

While we're at it, add the usual
`extension FooChecks on Subject<Foo>` boilerplate that we'll use in
those tests.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -21,6 +21,7 @@ class MessageListPage extends StatefulWidget {
 
   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));
   }
 

test/flutter_checks.dart

@@ -15,6 +15,15 @@ extension GlobalKeyChecks<T extends State<StatefulWidget>> on Subject<GlobalKey<
   Subject<T?> get currentState => has((k) => k.currentState, 'currentState');
 }
 
+extension RouteChecks<T> on Subject<Route<T>> {
+  Subject<RouteSettings> get settings => has((r) => r.settings, 'settings');
+}
+
+extension RouteSettingsChecks<T> on Subject<RouteSettings> {
+  Subject<String?> get name => has((s) => s.name, 'name');
+  Subject<Object?> get arguments => has((s) => s.arguments, 'arguments');
+}
+
 extension ValueNotifierChecks<T> on Subject<ValueNotifier<T>> {
   Subject<T> get value => has((c) => c.value, 'value');
 }