Update #18146.

Change-Id: Ib447aabae9f203a8b61fb8c984b57d8e2bfe69c2
Reviewed-on: https://go-review.googlesource.com/33894
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>