Suggested by Dave Cheney in golang.org/cl/20405.

Change-Id: I581c11ae80034cb6ebef3de976e8ae9484472322
Reviewed-on: https://go-review.googlesource.com/20453Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>