Re: [w3c/webcomponents] slotchange event should not bubble (#571)

I think its worth mentioning that `composed: false` not meaning "this event does not propagate through the composed tree" is kind of odd. Is there a compelling use case against making `composed: false` limited to the tree in which it was dispatched?

One case that @sorvell brought up yesterday when I asked about this was the ability to determine which slot an event from a non-shadow child propagated through by merely listening to events on that slot. The counter-argument to this is that you could listen on the shadow-root's host and check the result of `composedPath()` for the slot.

Anecdotally, I think that the situations in which an event isn't either (1) meant only for the object controlling a the tree where the event was dispatched (i.e., a custom element dispatching events on itself to inform a component wrapping it) or (2) acceptably handled by any ancestor in the composed tree are rare.

If it's improper for an event to propagate to ancestor trees of the target's root because exposing that event is potentially 'unsafe' (event name conflicts, carries internal information), then why isn't it also improper for those events to propagate into descendant trees given the contents of those trees are similarly unknown and potentially unsafe for those same events?

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