From: https://phabricator.services.mozilla.com/D14910#inline-85493

Now that we're using ES6 class syntax here, I guess this hack isn't necessary anymore? I think subclasses can just delegate to the parent class with super.receiveMessage.

Also, as part of the XBL removal effort, I guess all subclasses have been cleared anyways... is there a good reason to not merge this with the other receiveMessage method somehow?

Anyhow, maybe good fodder for a follow-up bug.

This structure was maintained in Bug 1492967 to make it more of an automated, less risky change. But now that there aren't separate widgets for remote and non-remote browsers, this is due for a cleanup. Let's wait for Bug 1441935 so we can do this in JS instead of XML.