gistfile1.js

var XFooPrototype = {
  createdCallback: function() {
    this.createShadowRoot({ mode: 'open' });
    new MutationObserver(() => distribute(this)).observe(this, { childList: true }); 
    distribute(this);
  }
}

function distribute(host) {
  var slots = host.shadowRoot.querySelectorAll('content');
  [].forEach.call(slots, empty); // empty slots (required?)
  distributeCandidates(host, host.children, slots);
}

function distributeCandidates(root, candidates, slots) {
  [].forEach.call(candidates, candidate => {
    if (candidate.tagName === 'CONTENT') {
      distributeCandidates(candidate.distributedNodes, slots);
      candidate.ondistributionchanged = () => distribute(root);
    } else {
      allocate(candidate, slots);
    }
  });
}

function allocate(candidate, slots) {
  for (var i = 0; i < slots.length; i++) {
    var selector = slots[i].getAttribute('select');
    if (!selector || candidate.matches(selector)) {
      slots[i].insertAt(candidate, slots[i].distributedNodes.length); // append
      return true;
    }
  }
}

function empty(content) {
  [].forEach.call(content.distributedNodes, node => content.remove(node));
}