Re: [webcomponents] Need "slotting changed" event (#288)

I don't think this should be an event, using an event is much heavier weight and doesn't really make sense since the event path itself depends on slotting, it also seems super weird to fire this event at microtask timing instead of just using MutationObserver which is the "observe the DOM changing at microtask" system that already exists. This is duplicating that notification system, but using way more CPU and power to do it. ex. if 10 slots change, this fires 10 events, which are very expensive. Dispatching 10 mutation records is massively cheaper.

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https://github.com/w3c/webcomponents/issues/288#issuecomment-185821977