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

It seems like the latest spec doesn't define when the fallback contents of a `slot` element is used.

Since the [CSS Scoping Module Level 1](http://www.w3.org/TR/css-scoping-1/) refers to the concepts of [distribution list](http://www.w3.org/TR/css-scoping-1/#distribution-list) and [insertion point](http://www.w3.org/TR/css-scoping-1/#insertion-point) (this should be renamed to slot), we should probably define that either a `slot` element without any distributed nodes doesn't constitute a `insertion point` (a.k.a. slot).

We could alternatively define that an insertion point can have an empty distribution list, and then make the CSS Scoping Module define the fallback behavior but that seems cumbersome.

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/317