buildText method
Implementation
T? buildText(DOMElement? domParent, T? parent, TextNode domNode,
    DOMTreeMap<T> treeMap) {
  if (domParent != null) {
    domNode.parent = domParent;
  }
  var text = getDOMNodeText(domNode);
  T? textNode;
  if (parent != null) {
    textNode = appendElementText(parent, text);
  } else {
    textNode = createTextNode(text);
  }
  if (textNode != null) {
    treeMap.map(domNode, textNode);
  }
  return textNode;
}