Fixes race builders, broken in https://golang.org/cl/16953

Change-Id: Id61171672b69d0ca412de4b44bf2c598fe557906
Reviewed-on: https://go-review.googlesource.com/17936
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>