Fix memory leaks in Hero widget (#147303)

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

@@ -425,7 +425,6 @@ class _HeroState extends State<Hero> {
 }
 
 // Everything known about a hero flight that's to be started or diverted.
-@immutable
 class _HeroFlightManifest {
   _HeroFlightManifest({
     required this.type,
@@ -455,8 +454,10 @@ class _HeroFlightManifest {
 
   Object get tag => fromHero.widget.tag;
 
+  CurvedAnimation? _animation;
+
   Animation<double> get animation {
-    return CurvedAnimation(
+    return _animation ??= CurvedAnimation(
       parent: (type == HeroFlightDirection.push) ? toRoute.animation! : fromRoute.animation!,
       curve: Curves.fastOutSlowIn,
       reverseCurve: isDiverted ? null : Curves.fastOutSlowIn.flipped,
@@ -505,6 +506,11 @@ class _HeroFlightManifest {
     return '_HeroFlightManifest($type tag: $tag from route: ${fromRoute.settings} '
         'to route: ${toRoute.settings} with hero: $fromHero to $toHero)${isValid ? '' : ', INVALID'}';
   }
+
+  @mustCallSuper
+  void dispose() {
+    _animation?.dispose();
+  }
 }
 
 // Builds the in-flight hero widget.
@@ -531,7 +537,13 @@ class _HeroFlight {
   late ProxyAnimation _proxyAnimation;
   // The manifest will be available once `start` is called, throughout the
   // flight's lifecycle.
-  late _HeroFlightManifest manifest;
+  _HeroFlightManifest? _manifest;
+  _HeroFlightManifest get manifest => _manifest!;
+  set manifest (_HeroFlightManifest value) {
+    _manifest?.dispose();
+    _manifest = value;
+  }
+
   OverlayEntry? overlayEntry;
   bool _aborted = false;
 
@@ -634,6 +646,7 @@ class _HeroFlight {
       _proxyAnimation.removeListener(onTick);
       _proxyAnimation.removeStatusListener(_handleAnimationUpdate);
     }
+    _manifest?.dispose();
   }
 
   void onTick() {

packages/flutter/test/widgets/heroes_test.dart

@@ -301,7 +301,10 @@ Future<void> main() async {
     expect(find.byKey(thirdKey), isInCard);
   });
 
-  testWidgets('Heroes still animate after hero controller is swapped.', (WidgetTester tester) async {
+  testWidgets('Heroes still animate after hero controller is swapped.',
+    // TODO(polina-c): remove when fixed https://github.com/flutter/flutter/issues/145600 [leak-tracking-opt-in]
+    experimentalLeakTesting: LeakTesting.settings.withTracked(classes: const <String>['CurvedAnimation']),
+    (WidgetTester tester) async {
     final GlobalKey<NavigatorState> key = GlobalKey<NavigatorState>();
     final UniqueKey heroKey = UniqueKey();
     final HeroController controller1 = HeroController();