The behavior here is a bit weird because Document is still not a
WebIDL object, so calling createNodeIterator or createTreeWalker via
an Xray will call the XPCOM versions of those methods. That means
that I can't just disable XPCOM-based wrapping for TreeWalker and
NodeIterator altogether, unfortunately, which means a web page could
try stashing a TreeWalker in something like userdata and then getting
it back and end up wrapping it as an XPCOM object the second time.
I could "fix" that by adding a wrapper cache and whatnot, I guess, if
desired... But the problem will go away once we convert Document in
any case.
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