Re: [whatwg/dom] Add declarative Shadow DOM features (#858)

Ok, I just force-pushed. Do I also need to fix the (pre-existing) issue with en-GB spelling of "serialisation", to get Travis to build?

```
en-GB spelling (use lang="en-GB", or <!-- en-GB -->, on the same line to override):
6922:<a href="https://www.w3.org/TR/html5/single-page.html#html-fragment-serialisation-algorithm">fragment
Makefile:22: recipe for target 'deploy' failed
make: *** [deploy] Error 1
The command "make deploy" exited with 2.
```

As for implementer interest and @rniwa's last comment:
> > b. In the "backwards" case, the `attachShadow()` call from the component "wins" the race. Then the declarative content finishes parsing, and the parser tries to attach the declarative shadow root. In this case, that attachment will fail, and the declarative contents will be discarded. Here again, not performance-ideal, but everything keeps working correctly.
> 
> I don't think this behavior is acceptable. There needs to be a way for a component to let the declarative content arrive into its shadow root. Maybe we need an option to attachShadow which indicates whether the declarative content could be inserted or not.

I believe this is exactly [issue 871](https://github.com/w3c/webcomponents/issues/871), to expose shadowRoot on element internals. I would like to take up that issue also, but separately, and it sounds like we're close to agreement on that thread.

> Also, this poses another problem. Since the component would never know when the children had finished parsing, there is no point in which the component is safe to attach a shadow root until your sibling or your ancestor's sibling starts appearing. Having to write that kind of code manually everywhere seems very fragile & developer hostile.

This second point is exactly [issue 809](https://github.com/w3c/webcomponents/issues/809), to add a callback when parsing is "done". I also think we need to work on that issue, but again, separately. I'm in favor of a solution to that problem, and that is @rniwa's own issue, so presumably he agrees.

Maybe it is my optimistic reading of the above, but it seemed like we might be close. I suppose we will need to wait for @rniwa to return to find out?

In the meantime, what are Mozilla's thoughts on this API? One of your comments requested more clarity about the interaction between declarative and imperative shadow roots. Hopefully that's clear with this PR and the corresponding [HTML PR](https://github.com/whatwg/html/pull/5465)?

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