codereview: release-branch.go1.4 is now a work branch

(That is, changes can originate in the branch.
The main branch has diverged enough that this
may be necessary.)

LGTM=bradfitz
R=adg, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/187810043
[release-branch.go1.4] cmd/api: make API check fail for undeclared API in release branch

We forgot to do the usual API review.
Make that not possible in the future.
I'll pull this change over to the main
branch too, but it's more important
(and only testable) here.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/185050043