gc: test that asserts closures are not wrapped when they don't have closure vars.
gc: test that asserts closures are not wrapped when they don't have closure vars.

R=rsc, bradfitz
CC=golang-dev
http://codereview.appspot.com/5529060
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go