[webcomponents] Composed child not correctly defined (#383)

Since the `ShadowRoot` is not actually part of the composed tree, it should not be a composed child. Also, this does not seem to account for distribution.

I thought the composed tree was the tree where everything is distributed, i.e., what CSS operates on.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/383