Used by callMethod to get the JS object for each argument passed if the argument is a Dart class instance that delegates to a DOM object. See wrap_jso defined in dart:html.

Source

unwrap_jso(dartClass_instance) {
  try {
    if (dartClass_instance != null)
      return dartClass_instance is NativeFieldWrapperClass2 ?
          dartClass_instance.blink_jsObject : dartClass_instance;
    else
      return null;
  } catch(NoSuchMethodException) {
    // No blink_jsObject then return the dartClass_instance is probably an
    // array that was already converted to a Dart class e.g., Uint8ClampedList.
    return dartClass_instance;
  }
}