anchor.diff:      _listeners.add(listenerEntry);
       try {
         assert(_debugSetCanAddListeners(false));
  -      if (fireImmediatly) {
  +      if (fireImmediately) {
           listener(state);
         }
       } catch (err, stack) {
  
positive.code: 
    /// Subscribes to this object.
    ///
    /// The [listener] callback will be called immediatly on addition and
    /// synchronously whenever [state] changes.
    ///
    /// To remove this [listener], call the function returned by [addListener]:
    ///
  
negative.code:   /// Listeners cannot add other listeners.
    ///
    /// Adding and removing listeners is O(1).
    RemoveListener addListener(
      Listener<T> listener, {
      bool fireImmediately = true,
    }) {
      assert(() {
        if (!_debugCanAddListeners) {
          throw ConcurrentModificationError();
  