IsHiddenClosure is more descriptive.

Change-Id: I06651072925a958b148b64ab0db3a9bfc839af9b
Reviewed-on: https://go-review.googlesource.com/32224
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>