Unnecessary new (#20138)

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests
diff --git a/examples/flutter_gallery/test_memory/back_button.dart b/examples/flutter_gallery/test_memory/back_button.dart
index bbd0660..c8f2e11 100644
--- a/examples/flutter_gallery/test_memory/back_button.dart
+++ b/examples/flutter_gallery/test_memory/back_button.dart
@@ -30,7 +30,7 @@
 Future<void> main() async {
   runApp(const GalleryApp());
   await endOfAnimation();
-  await new Future<Null>.delayed(const Duration(milliseconds: 50));
+  await Future<Null>.delayed(const Duration(milliseconds: 50));
   debugPrint('==== MEMORY BENCHMARK ==== READY ====');
-  WidgetsBinding.instance.addObserver(new LifecycleObserver());
+  WidgetsBinding.instance.addObserver(LifecycleObserver());
 }