revert a type change

lib/test_reflective_loader.dart

diff --git a/lib/test_reflective_loader.dart b/lib/test_reflective_loader.dart
index 77a2447..6237f67 100644
--- a/lib/test_reflective_loader.dart
+++ b/lib/test_reflective_loader.dart
@@ -254,7 +254,8 @@
 Future<Object?>? _runFailingTest(ClassMirror classMirror, Symbol symbol) {
   bool passed = false;
   return runZonedGuarded(() {
-    return Future.sync(() => _runTest(classMirror, symbol)).then((_) {
+    // ignore: void_checks
+    return Future.sync(() => _runTest(classMirror, symbol)).then<void>((_) {
       passed = true;
       test_package.fail('Test passed - expected to fail.');
     }).catchError((e) {