that it drops from the expansion (like define/public) by
adding them to the origin syntax property (and sometimes
to disappeared-use; see the add-decl-props function
for details on those that aren't in the origin property)
this means that check syntax will now pick them up
so they'll show up in the blue boxes in drracket
Thanks Matthew, for some helpful advice and
comments on an initial version of the commit.
52f18ee370