Preparation step for types2 package.
Change-Id: I8f9557b1a48ad570ba38aac7b720e639218dc6a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/263623
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>