By using these utility functions, the code can be made a little shorter.
Thanks to Omar Shafie for pointing this out in
https://golang.org/cl/27393/.

Change-Id: I33fd97cf7d60a31d0844ec16c12bba530dcc6f6d
Reviewed-on: https://go-review.googlesource.com/32120
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>