Fix leaky tests. (flutter#154847)

Author: polina-c

packages/flutter/test/widgets/heroes_test.dart

@@ -2399,6 +2399,7 @@ Future<void> main() async {
       ),
     );
     final HeroController controller = HeroController();
+    addTearDown(controller.dispose);
 
     await tester.pumpWidget(
       MaterialApp(
@@ -2524,6 +2525,7 @@ Future<void> main() async {
       ),
     );
     final HeroController controller = HeroController();
+    addTearDown(controller.dispose);
 
     await tester.pumpWidget(
         MaterialApp(