[w3c/webcomponents] To notify mutation observers should make a copy of signalList before delivering mutation records (#518)

In [**notify mutation observers**](https://dom.spec.whatwg.org/#notify-mutation-observers), step 4 (making a copy of `signalList`) should happen before step 3 (executing a compound microtask subtask to deliver mutation records) for consistency.

Otherwise, all slot changes made while invoking mutation observers would be dispatched in step 6 even though the logical ordering would first delivering any pending mutation records to other mutation observers before doing so.

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