Re: [whatwg/dom] replaceChild() has odd mutation observer behavior after #754 (#814)

_child_'s previous sibling is saved, but not _node_'s. The first mutation event results from step 13 of [replace](https://dom.spec.whatwg.org/#concept-node-replace)

> Insert _node_ into _parent_ before _referenceChild_ with the _suppress observers flag_ set.

which eventually goes to step 7.1 of [insert](https://dom.spec.whatwg.org/#concept-node-insert)

> Adopt _node_ into _parent_’s node document.

which goes to step 2 of [adopt](https://dom.spec.whatwg.org/#concept-node-adopt)

> If _node_’s parent is non-null, then remove _node_.

which fires a mutation event. At that time, _node_'s previous sibling has already been removed, so the fired mutation event does not contain a _previousSibling_.

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The second mutation event in the array is a result of "queue a tree mutation record" in [replace](https://dom.spec.whatwg.org/#concept-node-replace) itself. That works fine.

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