Re: [dom] Fix the adoption step when replacing node by child into parent (#121)

I haven't really used `MutationObserver` so that I could say which is better from the developer's point of view, but a single record seems OK. Does the proposed fix address all cases, or is it possible for *node*'s parent to be *parent* with something like `a.replaceWith(b)` where `a` and `b` are siblings? Any mutation records created by adopting in that case seem equally redundant.

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