[racket] circular lists are not lists or sequences

I have some code that's been broken somewhere between 5.1.1 and git 
HEAD.  The issue seems to be either that circular lists no longer are 
considered lists by `list?', or `in-list' has added a `list?' check (and 
circular lists were never `list?').

In any case, these are the kinds of things I would expect to work, but 
don't.  Is this a bug?

 > (for/first ([y (shared ((x (cons 1 x))) x)]) y)
for: expected a sequence for y, got something else: #0='(1 . #0#)
   context...:
 
/Users/dvanhorn/Documents/git/racket/collects/racket/private/for.rkt:451:2: 
make-sequence
 
/Users/dvanhorn/Documents/git/racket/collects/racket/private/misc.rkt:87:7

 > (for/first ([y (in-list (shared ((x (cons 1 x))) x))]) y)
in-list: contract violation
   expected: list?
   given: #0='(1 . #0#)
   context...:
 
/Users/dvanhorn/Documents/git/racket/collects/racket/private/for.rkt:551:2: 
in-list
 
/Users/dvanhorn/Documents/git/racket/collects/racket/private/misc.rkt:87:7

Thanks,
David