Introduced in https://golang.org/cl/20462

Change-Id: I6c55f87228f0980058a10f204b57ead7594e3492
Reviewed-on: https://go-review.googlesource.com/20490Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-on: https://go-review.googlesource.com/22075Reviewed-by: Minux Ma <minux@golang.org>