1. Deprecated("Internal Use Only")
JsFunction wrap_event_listener(theObject, Function listener)

Source 

@Deprecated("Internal Use Only")
js.JsFunction wrap_event_listener(theObject, Function listener) {
  var thisHashCode = theObject.hashCode;
  var listenerHashCode = identityHashCode(listener);

  _knownListeners.putIfAbsent(thisHashCode, () => new Map<int, js.JsFunction>());
  _knownListeners[thisHashCode].putIfAbsent(listenerHashCode, () =>
    new js.JsFunction.withThis((theObject, event) => listener(wrap_jso(event))));

  return _knownListeners[thisHashCode][listenerHashCode];
}