Re: [webcomponents] slot element's fallback contents is never used (#317)

I'm indifferent with regards to wether we support fallback or not but the spec should be consistent.  If we're getting rid of the concept of fallback then, [the definition of `slot` element](https://w3c.github.io/webcomponents/spec/shadow/#the-slot-element) should not refer to it.

@travisleithead, @annevk: do you have any opinions about this feature?

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