Re: [whatwg/dom] Why is the second parameter to MutationObserver.observe() optional? (#332)

Per IDL you cannot have mandatory dictionaries as final argument. They are treated identically to {}. That is a little weird if the dictionary ends up being effectively required as is the case here, but marking it mandatory doesn't really help us since we still have to handle the {} case anyway.

See https://github.com/heycam/webidl/issues/130 for more discussion.

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