dl: add go1.13beta1
dl: add go1.13beta1

Change-Id: I44288dfd6a3193f5cc292ae9b14c8c9ee45e3933
Reviewed-on: https://go-review.googlesource.com/c/dl/+/183977
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/dl