Fix that flutter test does not understand concurrency (#125942)

Close flutter/flutter#125940

I will add tests if this PR looks roughly OK :)

The fix mainly mimics flutter/flutter#115160 - just remove the default argument.

p.s. I ran into this bug when wanting to set concurrency in my dart_test.yaml for one set of my tests which I need to be executed without parallalization.

Author: fzyzcjy