[dom] Node.replaceChild() doesn't correctly queue mutation records

nox has just created a new issue for https://github.com/whatwg/dom:

== Node.replaceChild() doesn't correctly queue mutation records ==
Step 13 states that the mutation record's previous sibling should be 
`child`'s previous sibling, but `child` was removed from its parent on
 step 10. There should be a step between 9 and 10 where its previous 
sibling is bound to a variable and reused in step 13.

See https://github.com/whatwg/dom/issues/59