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

> 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?

I thought this problem a while ago, and I have decided the current design. I am not sure I can explain well what I thought, but let me try:

- For an ancestor tree, the internal structure of descendant tree should not be a concern because it is the implementation detail of a component.
- For a slot in a descendant tree, which elements in ancestor trees are slotted into it should be a concern. It is not the implementation details of other components. It should not be hidden.

In other words:
-  A boundary of [shadowHost (ancestor tree) ===> shadow-root (descendant tree)] would be a separation of a concern.
- A boundary of [slot (descendant tree) ===> slotted elements (ancestor tree)] should be *open*.




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