The suggested fix does not really work. `FileSystemObject` is an abstract class. One is going to have to create either `FileInfo` or a `DirectoryInfo` object -- you pick the one that has the functionality you need. For this purpose, I left 'as-file` as-is and added `as-dir`.
In addition, as a rough approximation to `clojure.java.io/file`, I added `file-info` and `dir-info`. They will take multiple arguments, stringify them, and join them with the directory separator char interposed. It will convert any 'wrong' separator chars to the correct one for the platform -- once you enter the land of `DirectoryInfo` and `FileInfo`, it is hard to remain agnostic. Thus you could do something mixed mode like `(file-info "a/b" "c\\d")`.
This will be in the alpha6 of 1.12.0 when it comes out, probably later this week.