For future use in rewrite rules.

Change-Id: Ic9875beb0dea6e0bbcbd4b75d99a53f4a9a7c3fd
Reviewed-on: https://go-review.googlesource.com/101275
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>