[h-e-w] expand-file-name leaves forward slashes

[Stephen Leake]

I'm now the ada-mode maintainer, and I'm testing the various ada-mode
functions on "pure Windows"; using the DOS shell instead of Cygwin.

It's not working, because all of the filenames have forward slashes as
directory separators!

To be specific, this:

(expand-file-name "c:/foo/bar")

is supposed to return

"c:\foo\bar"

But it's returning

"c:/foo/bar"

I'm using Emacs 21.3.1, and starting it from a Windows icon that runs
runemacs.exe. 

I have Cygwin installed, but I've made sure it's _not_ in PATH when
this Emacs runs.

directory-sep-char is set to '/', which is another symptom of the same
problem. I also noticed that directory-sep-char is now deprecated;
what are we supposed to use instead? I hate it when the documentation
says something is deprecated, but gives no clue about the replacement.

Any suggestions on why this is happening, and how to fix it?

-- 
-- Stephe