api: promote next to go1.12

Change-Id: I9a30c76d1299a494cce69b1060156bc2932ac8d5
Reviewed-on: https://go-review.googlesource.com/c/154857Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>