So apparently defining position has been deprecated since 1.29. Who knew?
I've found out about this after seeing https://phabricator.wikimedia.org/T184257 in the #wikimedia-tech wikibugs feed, and was able to trace this back to patch https://gerrit.wikimedia.org/r/#/c/320315/ (should be it, anyways).
We'll need to remove this from both skins and extensions at some point.
NOTE: Any future registered ResourceLoader modules should not have this annotation in their definition.