Fails on builder for unknown reasons.

Fixes #18198.
Update #18146.

Change-Id: Iaa85826655eee57d86e0c73d06c930ef3f4647ec
Reviewed-on: https://go-review.googlesource.com/33906
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>