It appears very unlikely that we could throw a circularity error
because of this race condition (as noted in the description, the
placeholders for each class in the chain would have to be removed).
However, it seems likely we could get a heap verification error, as
we'll violate the invariant that all klasses in the heap are either
in the system dictionary or placeholder tables.
This bug needs to be examined at the same time 4699981 is looked at,
and when we look at class loading to see if it can be made more parallel.