[webcomponents] assignedSlot should be defined on Node (#310)

It seems that there is nothing that prevents a comment node, ProcessingInstruction, etc... to be distributed into a slot so I don't think it makes sense to define `assignedSlot` on `Element` and `Text` interface separately.  We should just define it on `Node` instead.  And make `DocumentFragment`, `Document`, etc... return null by the virtue of them never being distributed to any slot.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/310