spec: 9 only looks prime
spec: 9 only looks prime

Fixes issue 2870.

R=golang-dev, agl, lvd
CC=golang-dev
http://codereview.appspot.com/5618050
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go