Andre wrote https://golang.org/cl/13454043 (never submitted),
which served as the basis for Ross Light's https://golang.org/cl/19235.

Individual CLA verified by hand.

Change-Id: Ic09e8efd84b7ded3ae472c204133e40cb85d97f7
Reviewed-on: https://go-review.googlesource.com/23377
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>