The code uses the filename suffix instead of the bool parameter to
determine what to do.

Fixes #19474.

Change-Id: Ic552a54e50194592a4b4ae7f74d3109af54e6d36
Reviewed-on: https://go-review.googlesource.com/38265
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>