Suggested during code reviews of last 15 CLs (or so).

Change-Id: If780f6eb47a7a31df133c64d5dcf0eaf04d8447b
Reviewed-on: https://go-review.googlesource.com/10675Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>