ASN.1 has an private class, but current implementation does not support it.

Change-Id: I3ebf07a048831869572f75223cb17d4c115caef7
GitHub-Last-Rev: b3c69ad091218acfa0bb0e34111cceae69586eb9
GitHub-Pull-Request: golang/go#25195
Reviewed-on: https://go-review.googlesource.com/110561
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>