One possible issue here: if all nodes in selected are nested ones (thus should be filtered out), we end up outputting empty content, but in that case we should actually use the fallback content (outlet.childNodes).

According to http://w3c.github.io/webcomponents/spec/shadow/#dfn-pool-distribution-algorithm, the algorithm here should be:

• For each direct child node of raw, check if it matches select.
• If yes, push it into selected.
• If selected ends up empty, use outlet.childNodes instead.

This also means we need a matches() helper in util/dom.js:

var p = Element.prototype
var matches = p.matches || p.webkitMatchesSelector || p.mozMatchesSelector || p.msMatchesSelector
exports.matches = function (el, selector) {
  return matches.call(el, selector)
}

I wanted to avoid element.matches because its browser support is slightly worse than querySelectorAll (http://caniuse.com/#feat=matchesselector vs http://caniuse.com/#search=queryselectorall). In practice it probably doesn't matter (opera mini 8 is the only browser that's still used of those, and I don't know if vue even supports it), but I decided to play it safe.