Mock object not recognized as Mockito in Flutter widget test using GetIt

Question

I am writing a widget test that verifies a logout button. When the logout function is triggered via a user button tap, some items in FlutterSecureStorage are cleared. I want to verify that a method was called in our Preferences class which is a wrapper for FlutterSecureStorage. We use GetIt as a dependency injector in the app and for tests.

In my widget test, I have declared a Mock for our Preferences class as follows:

class MockPreferences extends Mock implements IPreferences {
  @override
  Future<void> clearPreferences() async {
    // clear some values
  }
}

In my widget test setup I have the following:

GetIt.instance.registerSingleton<IPreferences>(MockPreferences());

And in my testWidgets I am obtaining a reference to the mock as follows:

var prefs = GetIt.instance<IPreferences>();

When my widget test runs, when I place a break point after the line above, I can verify that the prefs object is definitely of type MockPreferences and other interactions with the widget test prove that methods are being called on the mock object and not the real object.

When I try to verify that the clearPreferences() method was called using Mockito's verify function as follows:

  var prefs = GetIt.instance<IPreferences>();
  verify(prefs.clearPreferences()).called(greaterThan(0));

The following TestFailure object was thrown running a test: Used a non-mockito object. 

Placing a breakpoint after the var prefs line I can see the the prefs object IS a mockito object, but the test will not recognize it as a mockito object and the test fails.

Can anyone offer any suggestions? Thanks!