They were almost identical.
Merge them and some of their calling code.

Passes toolstash -cmp.

Change-Id: I9e92a864a6c09c9e18ed52dc247a678467e344ba
Reviewed-on: https://go-review.googlesource.com/26754
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>