Re: [w3c/webcomponents] How to wait for will-be custom elements to be upgraded? (#558)

> Another thing is that if Custom Element classes are registered after the elements already exist in DOM, then their `attributeChangedCallback` methods will not be fired with the initially set attribute values (if any).

This is explicitly not the case in v1 API.  We would always invoke `attributeChangedCallback` on every attribute including ones that were present at the time of upgrade.  If that's not the case, then we need to file a spec bug and fix that.

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