api: add recent Go 1.13 work

Change-Id: I0d1048723057dff0caf2059ce2efd63eb953ed82
Reviewed-on: https://go-review.googlesource.com/c/go/+/173947
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>