Same idea as golang.org/cl/20322.

Passes toolstash/buildall.

Change-Id: Ida5960e57ff6e1e8de75c2720f21aeae56a8e415
Reviewed-on: https://go-review.googlesource.com/20352
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>