sub-repo.

It's a straight copy, except for these modifications:
* "exp/html" and "exp/html/atom" imports were renamed, and
* the "TODO... When this package moves out of exp" comment was
  deleted from atom/atom.go.

The matching change is at https://golang.org/cl/7317043

The rationale was discussed at
https://groups.google.com/d/topic/golang-nuts/Qq5hTQyPuLg/discussion

R=adg, remyoudompheng, dave
CC=golang-dev
https://golang.org/cl/7310063