Testing for bugs off master for opaque-keys deprecation

This pull request is to identify trouble spots after opaque-keys deprecation.
This fill_in_run function should only ever get called from within base.py (or split.py or whatever), right?  Could we mention that more explicitly in the docstring (I had a paranoid moment where I was afraid we were having to manually call this within djangoapp code, which would be bad, etc).