Similar idea to golang.org/cl/28970.

Change-Id: I9d2feb1a669d71ffda1d612cf39ee0d3c08d22d2
Reviewed-on: https://go-review.googlesource.com/29357
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>