Extract out some common boiler plate logic.

Passes toolstash-check -all.

Change-Id: Iddc8a733af8262558f56d13c91d9c27ee0d61330
Reviewed-on: https://go-review.googlesource.com/40253
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>