$wgUser will be used is the User object corresponding to the user name passed in the "user" parameter. This avoids calling User::newFromName() (which will unstub $wgAuth, do database requests, etc.) when an object is already available.
Change-Id: Ia6c5ee1a59581b6410232fa3101af5ae1ff8577e
(Almost looks like it could all go into ResourceLoaderModule... But that uses a different version, seemingly, the only one. 3 other subclasses of ResourceLoaderModule implement the same version of getFlip as is moved into a parent class here... Seems daft to have a different version in the base abstract class... Minor oversight?)
Some documentation