The problem here is that we have separate bootstrap compiler and
artifacts (stdlib, for example) that are used in tests and implicitly
participate in them.
For example, this test started to fail when bootstrap compiler was
advanced to a version where pre_release flag is enabled. As a result,
dependent stdlib become pre_release as well and now we have additional
error about `Unit` type
This has no effect for the original use case of mock class descriptors
(unresolved classes used in libraries), but is necessary for pre-release
classes in case the compiler is a release, because such classes end up in scope
and can be the result of the resolution
Dmitriy Novozhilov