gosrc: regenerate with Go 1.12

This change regenerates the list of standard packages to contain
packages that are in Go 1.12, which has been recently released.

It was done by running:

	go generate ./gosrc

Updates golang/gddo#603

Change-Id: I641eeb60c5a47feaa93dc1620ee9ad96ed6878ca
Reviewed-on: https://go-review.googlesource.com/c/164301
Reviewed-by: Tuo Shan <shantuo@google.com>