anchor.diff:    XmlName get name => prefix == null || prefix.isEmpty
         ? new XmlName(_XMLNS)
         : new XmlName(prefix, _XMLNS);
  -
  -  void use() {
  -    used = true;
  -  }
   }
   
   abstract class _XmlNodeBuilder {
  
positive.code:  * A builder to create XML trees with code.
   */
  class XmlBuilder {
    final List<_XmlNodeBuilder> _stack =
        new List.from([new _XmlDocumentBuilder()]);
  
    /**
     * Adds a [XmlText] node with the provided [text].
  
negative.code:     if (prefix == _XMLNS || prefix == _XML) {
        throw new ArgumentError('The "$prefix" prefix cannot be bound.');
      }
      if (optimizeNamespaces && _stack.any(
          (builder) => builder.namespaces.containsKey(uri) &&
                       builder.namespaces[uri].prefix == prefix)) {
        // namespace prefix already correctly specified in an ancestor
        return;
      }
      if (_stack.last.namespaces.values.any((meta) => meta.prefix == prefix)) {
  