copy method
Returns a copy if this instance.
Implementation
@override
DOMTemplate copy({bool resolveDSX = false}) {
  if (resolveDSX) {
    var dsx = asDSX;
    if (dsx != null) {
      var s = dsx.createResolver().resolveValueAsString();
      return DOMTemplateContent(s);
    }
  }
  var copy = DOMTemplateBlockVar(variable);
  return copy;
}