void allowTagExtension(String tagName, String baseName, { UriPolicy uriPolicy, Iterable<String> attributes, Iterable<String> uriAttributes })

Allow custom tag extensions with the specified type name and specified attributes.

This will allow tag extensions (such as <div is="x-foo"></div>), but will not allow custom tags. Use allowCustomElement to allow custom tags.

Source 

void allowTagExtension(String tagName, String baseName,
    {UriPolicy uriPolicy,
    Iterable<String> attributes,
    Iterable<String> uriAttributes}) {
  var baseNameUpper = baseName.toUpperCase();
  var tagNameUpper = tagName.toUpperCase();
  var attrs = attributes
      ?.map /*<String>*/ ((name) => '$baseNameUpper::${name.toLowerCase()}');
  var uriAttrs = uriAttributes
      ?.map /*<String>*/ ((name) => '$baseNameUpper::${name.toLowerCase()}');
  if (uriPolicy == null) {
    uriPolicy = new UriPolicy();
  }

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