It was a bit weird to have it at the top of pgen.go.
This does half of the TODO at the top of the comment.

Change-Id: I65140fa05673b2dbb6feddb8c1877f6d624a7844
Reviewed-on: https://go-review.googlesource.com/40698
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>