api: promote next to go1.11

Change-Id: Ib8fa0a12363993033201ff707c315f4030811f89
Reviewed-on: https://go-review.googlesource.com/120595
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>