(Split out from CL 22243.)

Change-Id: Idac1748c8db2b2ed0484e4afadb105c471c6ce34
Reviewed-on: https://go-review.googlesource.com/22321Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>