 {
  var baseNameUpper = baseName.toUpperCase();
  var tagNameUpper = tagName.toUpperCase();
  var attrs = attributes
      ?.map&lt;String&gt;((name) =&gt; &#39;$baseNameUpper::${name.toLowerCase()}&#39;);
  var uriAttrs = uriAttributes
      ?.map&lt;String&gt;((name) =&gt; &#39;$baseNameUpper::${name.toLowerCase()}&#39;);
  if (uriPolicy == null) {
    uriPolicy = new UriPolicy();
  }

  add(new _CustomElementNodeValidator(uriPolicy,
      [tagNameUpper, baseNameUpper], attrs, uriAttrs, true, false));
}<