Because of the way replaceChild is defined (https://dom.spec.whatwg.org/#dom-node-replacechild),
it should add two mutation records when the inserted node was originally a sibling of the reference child.
e.g. node.replaceChild(node.lastChild, node.firstChild) when node has two children.
This bug was found by the newly added test: LayoutTests/http/tests/w3c/dom/nodes/MutationObserver-childList.html
*** This bug has been marked as a duplicate of bug 149546 ***