WIP: https://codereview.chromium.org/2789363002

Making this avaialble to someone in Blink>CSS.

FYI, in the previous attempt I tried to apply fallback contents in
document (i.e. slot element is in document tree), but now you don't have to.
So the code should be simpler.

Looks approachable, especially with kochi's WIP patch.

Re comment#4
the discussion on https://github.com/w3c/webcomponents/issues/631
may sound slot's children in document tree should also
return its fallback contents, but the latest HTML spec says:

https://html.spec.whatwg.org/multipage/scripting.html#dom-slot-assignednodes
The assignedNodes(options) method, when invoked, must run these steps:
 1. If the value of options's flatten member is false, then return this element's assigned nodes.
 2. Return the result of finding flattened slotables with this element.

So "2." has to be applied for assignedNodes({flatten: true}) case,
and "finding flattened slotables" algorithm says:

https://dom.spec.whatwg.org/#find-flattened-slotables
To find flattened slotables for a given slot slot, run these steps:
 1. Let result be an empty list.
 2. If slot’s root is not a shadow root, then return result.
 ...

So "2." applies in the document tree, thus slot.assignedNodes({flatten:true})
returns an empty array when the slot is in a document tree.

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