The use of a prefix check was too liberal.
Noted in review after submit.

Change-Id: I4fe1df660997efd225609e818040b8392fab79f0
Reviewed-on: https://go-review.googlesource.com/23375
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>