This simplifies the rewrite rules.

Change-Id: Iff062297d42a23cb31ad55e8c733842ecbc07da2
Reviewed-on: https://go-review.googlesource.com/129377
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>