This is a followup to moving the ModuleEnv to the compiler directory and
making it immutable.

R=mtrofin@chromium.org, ahaas@chromium.org

Bug: 
Change-Id: I0f5ec1b697bdcfad0b4dc2bca577cc0f40de8dc0
Reviewed-on: https://chromium-review.googlesource.com/616762
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47419}