Remove default expectation for UserTag (#86877)

See: https://github.com/flutter/engine/pull/27646#issuecomment-885035046

packages/flutter/test/widgets/binding_first_frame_developer_test.dart

@@ -12,7 +12,11 @@ void main() {
   test('first frame callback sets the default UserTag', () {
       final WidgetsBinding binding = WidgetsFlutterBinding.ensureInitialized();
 
-      expect(developer.getCurrentTag().label, equals('Default'));
+      // TODO(iskakaushik): https://github.com/flutter/flutter/issues/86947
+      final String userTag = developer.getCurrentTag().label;
+      final bool isValid = <String>{'Default', 'AppStartUp'}.contains(userTag);
+      expect(isValid, equals(true));
+
       developer.UserTag('test tag').makeCurrent();
       expect(developer.getCurrentTag().label, equals('test tag'));