`MutationObserverInit.attributeFilter` can accept an iterator, not just an array

[trusktr]

Specifically this: microsoft/TypeScript#50031

I'm not clear on how to make that change here.

[saschanaz]

Yeah, unfortunately supporting this would complicate things. We can't just redefine the field in .iterable.d.ts. 😞

[saschanaz]

(At this point I wonder we should get a totally separate dom.lib.d.ts for ES2015+ support, rather than augmenting ES5 version of the lib.)