Also added go:generate directive.

Change-Id: Ib1f0eddc75e3c47a4d904786a29b964a35b18456
Reviewed-on: https://go-review.googlesource.com/21042Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>