[ShadyCSS] error scoping distributed content

`` has a `shadowRoot` that looks like this (notice it wraps the slot with a div, "for reasons"):
```html

```
and `` has a `shadowRoot` with a `` in it.

With ShadyCSS enabled, content distributed into `` is wrongly scoped to it, nullifying styles set from the scope above, e.g. http://jsbin.com/mipoci/1/edit?html,output
```html

.bg-color { background: orange; }

```
results in the div having the classes `bg-color style-scope outer-element` instead of just `bg-color`