As per the comments in golang.org/cl/78617. Also leaving a comment here,
to make sure noone else thinks to re-introduce the iota like I did.

Change-Id: I2a2275998b81896eaa0e9d5ee0197661ebe84acf
Reviewed-on: https://go-review.googlesource.com/78676
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>