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

I've revised the algorithms. 

The change of https://github.com/w3c/webcomponents/commit/96403c3b808aa581852497a25321440462351489 broke the assumption of:

- S == N.assignedSlot <===>  N is in the member of S.getAssignedNodes().

because it changed the behavior of `getAssignedNodes`.

Now https://github.com/w3c/webcomponents/commit/609a11e3f39ea4d1b3ba30460ae07d14dc8399ce supports the fallback content at the layer of *get distributed nodes algorithm*, instead of at the layer of *get assigned nodes* algorithm, this issue wouldn't happen.

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