A residue of the automatic translation, this closure is easily rewritten
to a simpler, smaller, and faster construct.

Discovered while analyzing #10269, which I still plan to fix.

Change-Id: I76b12290280d81880c446b4cf75da633a94482d4
Reviewed-on: https://go-review.googlesource.com/8270Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Minux Ma <minux@golang.org>