[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://golang.org/cl/185050043
cmd/dist: ignore \r in crlf EOL when splitlines()

Fixes build on Windows. Fixes #9234.

Change-Id: Iebf4317e7cc20ba1afea5558553166cd89783316
Reviewed-on: https://go-review.googlesource.com/1421
Reviewed-by: <iant@golang.org>