How are comment-body fields created in the D6->D8 migration

Hi IMP team
Looking for assistance on https://drupal.org/node/2228763
Basically we're adding a comment-type and this changes the format of comment bundles.
The migration elements are done but the MigrateDrupal6Test is passing because the body field isn't recognised for the bundle. I am guessing this is because the format is different, it was entity_Type__fieldname (eg node__comment) and now it will be the comment type id - default is comment.

So wondering if someone can point me to where the comment-body fields created in the migration so I can update them.

Thanks

Lee