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The method is defined at Node. Maybe just throwing an exception is feasible. But it would be inconsistent if we allow a cloned host element to clone its shadow, the behavior looks inconsistent.
Yeah, this one is interesting. I think I'll define just throwing an exception for now, and file a bug to follow up after we gather some implementation feedback.
http://dvcs.w3.org/hg/webcomponents/rev/769c949cd10c
Filed bug 15497 to track implementer feedback research.