There is no semantic change to this commit. A follow-up commit will
change this package's behavior, but this preparatory commit will make
the follow-up's diff smaller, and less noisy.

Change-Id: I12e356fc1f29d3c4a7c3374aab4a1b1eefe01144
Reviewed-on: https://go-review.googlesource.com/21631Reviewed-by: Andrew Gerrand <adg@golang.org>