Implement newly proposed DOM selection approach

There may be some possible improvements for selecting light-DOM nodes in a shadow-rooted element with no contained content element -- I also tried querying the whole DOM natively and traversing it instead of the wrapped tree (https://gist.github.com/jcmoore/69a49f887e03c1d2bfa1#file-queryselector2-js) but it was no faster than the original method, and probably had significant memory implications.
Here is some history:

- The Google JavaScript style  guide always requires curlies.
- Chrome and Blink style guides only requires them if the statement does not fit on one line.

I'm tempted to go back to the Google style guide and not use the Chrome one...