Makes godoc 37KB smaller.
For #6853.

Change-Id: I0bd6e40f2b4da193768a9fc2ce494cacf7e83c32
Reviewed-on: https://go-review.googlesource.com/19697
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>