Not documented.

Source

@DomName('Document.createElement')
Element createElement(String tagName, [String typeExtension]) {
  var newElement = (typeExtension == null) ?
    _blink.BlinkDocument.instance.createElement_Callback_1_(unwrap_jso(this), tagName) :
    _blink.BlinkDocument.instance.createElement_Callback_2_(unwrap_jso(this), tagName, typeExtension);

  var wrapped;

  if (newElement['dart_class'] != null) {
    wrapped = newElement['dart_class'];         // Here's our Dart class.
    wrapped.blink_jsObject = newElement;
  } else {
    wrapped = wrap_jso(newElement);
    if (wrapped == null) {
      wrapped = wrap_jso_custom_element(newElement);
    } else {
      wrapped.blink_jsObject['dart_class'] = wrapped;
    }
  }

  return wrapped;
}