Re: [webcomponents] Define the behavior when a custom element constructor returns an element with a parent node. (#412)

I don't think we need to do anything in the upgrade case because if the element had been re-parented to somewhere else, there is no way for us to fix it.  `IsSameValue` check is probably sufficient in that case.

For HTML parser, though, we need to be even more careful.  e.g. what happens if the constructor returned an element with non-zero attributes?  Or an element with a wrong prefix in its qualified name?  Or it returned an element with a child?

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