When main.main returns, the process exits, so there's no need to cancel contexts.
This CL was originally reviewed as
https://go-review.googlesource.com/c/go/+/106915
and then retried in
https://go-review.googlesource.com/c/go/+/148758
but then reverted due to an embarrassing sequence
of careless moves.
Change-Id: Icdee0650996a442023e030697f10d2c31fd5fdff
Reviewed-on: https://go-review.googlesource.com/c/148877
Run-TryBot: Alan Donovan <adonovan@google.com>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>