Since r82457 (rMW97a945864ad217c4), CSSMin::remapOne() expects the application to have defined a global function called wfExpandUrl(). Otherwise, it will not expand local URLs with absolute paths. This is not a good API, and it should not have passed code review.
It should be possible for the caller to specify a callback, preferably without using global state. This may require adding additional parameters (e.g. an array of options, or an options object) or making the methods non-static (in which case the caller would specify the callback using a setter method).