These changes were missed when going from 2 to 3 argstorage slots.
https://go-review.googlesource.com/20296/

Change-Id: I930a307bb0b695bf1ae088030c9bbb6d14ca31d2
Reviewed-on: https://go-review.googlesource.com/21841Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>