Make boolean looking things boolean.

Change-Id: I8d1c0a32b471412b25a72908c7da6458d7bbe65b
Reviewed-on: https://go-review.googlesource.com/20723
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>