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

According to the spec, the `slotchange` event [bubbles](https://dom.spec.whatwg.org/#queue-a-mutation-observer-compound-microtask). The spec also says that `slotchange` should [re-fire](https://dom.spec.whatwg.org/#signaling-slot-change) at a slot's `assignedSlot`. Both of these behaviors together result in some odd an unexpected behavior: if a slot is assigned to another slot, the inner slot will see a node being assigned twice.  See the console output here: http://jsbin.com/jexabaf/edit?html,output.

I propose that the re-firing at `assignedSlot` is ok, but the bubbling behavior should be removed.

Note, Canary's current implementation appears to follow the spec (`slotchange` bubbles). Safari's implementation does not (`slotchange` does not bubble).  

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