dl: add go1.12rc1
dl: add go1.12rc1

Change-Id: I3b345a8d723de65686c433c948b84aecc09621c1
Reviewed-on: https://go-review.googlesource.com/c/161902
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/dl