[webcomponents] Integrate events with "get the parent" model (#429)

https://dom.spec.whatwg.org/ now defines "get the parent" as a way to build up an event path. I think it would be great if we defined in Shadow DOM how node's "get the parent" algorithm would be altered to account for Shadow DOM.

If that requires more hooks than I accounted for please let me know.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/429