They were using the result type to look up the op, not the arg type.

Change-Id: I0641cba363fa6e7a66ad0860aa340106c10c2cea
Reviewed-on: https://go-review.googlesource.com/14469Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>