The current implementation of nsXULDataSource is not (?) being used. We _do_ need a XUL datasource, but the _only_ thing it needs to do is wrap the in-memory datasource, and then forbid updates once the document is done loading. This way, were sure not to get any accidental sharing.
Never mind. This is stupid.
BULK MOVE: Changing component from XUL to XP Toolkit/Widgets: XUL. XUL component will be deleted.