Implement a comment by Ralph Corderoy on CL 29754.

Change-Id: I22bbede211ddcb8a057f16b4f47d335a156cc8d2
Reviewed-on: https://go-review.googlesource.com/29756
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>