docs: function getDirectCustomChildren(element, inclusive) {
    if (!inclusive && element.nodeState !== undefined) return [];
  
    let childRegistry = getChildRegistry(element);
    let children = [];
  
    for (let name in childRegistry) {
      let child = [].concat(childRegistry[name]);
  
      child.forEach((c) => {
        if (isCustomElement(c))
          children.push(c);
        else
          children = children.concat(getDirectCustomChildren(c));
      });
    }
  
    return children;
  }