[MLton] OS.FileSys.isDir

[Michael Norrish]

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

The Basis documentation doesn't seem totally clear.  

It seems as if MLton is following the link and then categorising what
it looks at.  

This has the problem that if the link is dangling, then calls like
isDir etc return "no such file" type errors.  This behaviour is
annoying, and it seems to me that MoscowML's behaviour is better: if I
pull a string out of a dirStream, I feel as if I should be able to
categorise it with the functions in OS.FileSys without them telling me
the file doesn't exist.

Michael.