In this case we produce additional warnings when preloading
evaluates constants. Possibly we shouldn't be evaluating expressions
with side-effects during preloading, but for now just skip this
test.
Class constants from parents should always be directly reused,
rather than re-evaluated as a separate copy. Previously this used
to happen automatically, as we'd just inherit the class constant
entry from the parent class. With mutable data there may now be
a separate copy of the constant, so we need to use that copy
when updating constants. Otherwise we may evaluate the same
constant multiple times.
Closes GH-7658.