[webcomponents] Should :unresolved apply to elements that failed to upgrade? (#422)

According to the [current spec text](https://w3c.github.io/webcomponents/spec/custom/#unresolved-element-pseudoclass), `:unresolved` should only apply to custom elements on which the constructor had yet to be invoked upon.

This would mean that if `:unresolved` would stop applying to custom elements that had failed inside a constructor (e.g. resulted in an exception).

Also see the issue #421.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/422