Use const route for notAnnounced. (flutter#144050)

packages/flutter/lib/src/widgets/navigator.dart

@@ -138,7 +138,7 @@ enum RoutePopDisposition {
 /// The type argument `T` is the route's return type, as used by
 /// [currentResult], [popped], and [didPop]. The type `void` may be used if the
 /// route does not return a value.
-abstract class Route<T> {
+abstract class Route<T> extends _RoutePlaceholder {
   /// Initialize the [Route].
   ///
   /// If the [settings] are not provided, an empty [RouteSettings] object is
@@ -2896,10 +2896,9 @@ enum _RouteLifecycle {
 
 typedef _RouteEntryPredicate = bool Function(_RouteEntry entry);
 
-class _NotAnnounced extends Route<void> {
-  // A placeholder for the lastAnnouncedPreviousRoute, the
-  // lastAnnouncedPoppedNextRoute, and the lastAnnouncedNextRoute before any
-  // change has been announced.
+/// Placeholder for a route.
+class _RoutePlaceholder {
+  const _RoutePlaceholder();
 }
 
 class _RouteEntry extends RouteTransitionRecord {
@@ -2937,12 +2936,12 @@ class _RouteEntry extends RouteTransitionRecord {
   /// remove as a result of a page update.
   static const int kDebugPopAttemptLimit = 100;
 
-  static final Route<dynamic> notAnnounced = _NotAnnounced();
+  static const _RoutePlaceholder notAnnounced = _RoutePlaceholder();
 
   _RouteLifecycle currentState;
-  Route<dynamic>? lastAnnouncedPreviousRoute = notAnnounced; // last argument to Route.didChangePrevious
-  WeakReference<Route<dynamic>> lastAnnouncedPoppedNextRoute = WeakReference<Route<dynamic>>(notAnnounced); // last argument to Route.didPopNext
-  Route<dynamic>? lastAnnouncedNextRoute = notAnnounced; // last argument to Route.didChangeNext
+  _RoutePlaceholder? lastAnnouncedPreviousRoute = notAnnounced; // last argument to Route.didChangePrevious
+  WeakReference<_RoutePlaceholder> lastAnnouncedPoppedNextRoute = WeakReference<_RoutePlaceholder>(notAnnounced); // last argument to Route.didPopNext
+  _RoutePlaceholder? lastAnnouncedNextRoute = notAnnounced; // last argument to Route.didChangeNext
   int? lastFocusNode; // The last focused semantic node for the route entry.
 
   /// Restoration ID to be used for the encapsulating route when restoration is

packages/flutter/test/painting/system_fonts_test.dart

@@ -40,10 +40,7 @@ void main() {
   // TODO(polina-c): clean up leaks, https://github.com/flutter/flutter/issues/134787 [leaks-to-clean]
   LeakTesting.settings = LeakTesting.settings.withIgnored(classes: <String>['CurvedAnimation']);
 
-  testWidgets('RenderParagraph relayout upon system fonts changes',
-  // TODO(polina-c): dispose _NotAnnounced, https://github.com/dart-lang/leak_tracker/issues/218 [leaks-to-clean]
-  experimentalLeakTesting: LeakTesting.settings.withIgnored(classes: <String>['ValueNotifier<String?>', '_NotAnnounced']),
-  (WidgetTester tester) async {
+  testWidgets('RenderParagraph relayout upon system fonts changes', (WidgetTester tester) async {
     await tester.pumpWidget(
       const MaterialApp(
         home: Text('text widget'),