Add a special helper for its one external use.

This is in preparation for an upcoming CL.

Passes toolstash -cmp / buildall.

Change-Id: I9d3463792afe220cc4bc89269bdecf0279abd281
Reviewed-on: https://go-review.googlesource.com/20933
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>