[MLton] OS.FileSys.isDir

[Stephen Weeks]

> Under MoscowML, isDir returns false for symbolic links that are
> links to directories.  Under MLton, isDir returns true.  Which is
> correct?

Hmmm.  All the implementations of SML that I have, including Moscow
ML, return true.  The MLton implementation of OS.FileSys.isDir uses
the Posix stat() system call followed by S_ISDIR, just as I expect the
other implementations do when on a Unixish system.  I can't imagine
how returning false is the right behavior.  If you want to look at a
symbolic link, use Posix.FileSys.lstat.