Hardcoded the limit on constants only allowed.

Change-Id: Idb9b07b4871db7a752a79e492671e9b41207b956
Reviewed-on: https://go-review.googlesource.com/13257Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>