Revert "fix mirror lub equality assertion"

This reverts commit d306a892d853197fbf1a7621227ca5ea4c48db65.

Turns out this is incorrect: https://github.com/mirah/mirah/commit/d306a892d853197fbf1a7621227ca5ea4c48db65#commitcomment-5345748
change mirror assertion from equality to same type check
use the pend w/ block form so that when ClassPath is added, it'll fail