I would not rely on `source-map`’s `util.join`. I do not consider it that stable. Moreover, it deals with _urls_. Which should work with unix paths, though. But here we need to consider Windows paths, too. Besides, we don’t need all the things that `util.join` does.

I’d say wrap the line in a `if (!isAbsolute(sourceMapFile)) {...}` block. I think the proper way to test if a path is absolute is by doing `path.resolve(p) === path.normalize(p)`.