[webcomponents] Custom elements' upgrade order is ambiguous in the case of a detached node (#420)

When a custom element is created but not in any document tree or its shadow-inclusive descendants, the order in which those elements are upgraded is not well defined with the following definition:

https://w3c.github.io/webcomponents/spec/custom/#dfn-upgrade-candidates-map
> Each registry has an associated upgrade candidates map of all instances of unresolved elements, mapping a custom element type to a sorted element queue. It is is initially empty.

https://w3c.github.io/webcomponents/spec/custom/#dfn-sorted-element-queue
> In addition to an element queue, there is also a sorted element queue. The custom elements are kept in the order of increasing custom element order.


---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/420