Proper way to handle cleanup tests in dart

Since dart test is parallel, I have encountered a problem that two more classes used same folder can't be deleted properly.

Let's see a folder structure:

Utility
- a_utility
- b_utility

Because Utility might be empty, AUtilityClasss will auto create Utility and sub-direcoty a_utility, and will be cleanup in teardown(). Hence, I remove Utility in teardown() to be sure that next test will not be dependency with previous.

But dart test is parallel by default, If there is two class(BUtilityClasss, CUtilityClasss, ...) that rely on same directory, teardown() may cause the error (Error: Can't delete because of in used, or Already Deleted) and might also affect tests in other b_utiliy_test.dart to fail. The current workaround I tried to prevent this situation is to assign -j1 argument, and there is no two test in parallel, so It will be safe.

Therefore, It cames to two question.

1. How to test that two classes use same directory?
2. Reconstruct the classes in one class to fit the test?

I can't find any discussion anywhere, any information is appreciated!