title: How to add to an existing list - got Unsupported operation: Cannot add to an unmodifiable list
body: <p>I am trying to add to a <code>List</code> which results in <code>Unsupported operation: Cannot add to an unmodifiable list</code></p>
  <p>But message list is not final, why can't I add Items to it?</p>
  <pre><code>class Chat {
    final String uuid;
    final User receiverUser;
    List&lt;Message&gt; messages;
  
    Chat({this.uuid, this.receiverUser, this.messages = const []});   
  }
  </code></pre>
  <hr />
  <p>In my provider I have a send message method</p>
  <pre><code>  void send(String receiverUUID) async {
      Message message = Message(
          message: messageController.text,
          receiverUUID: receiverUUID,
          timestamp: DateTime.now());
  
      await _messageApi.send(message);
  
      Chat chat = await _findOrCreate(receiverUUID);    
      /// FIXME Unhandled Exception: Unsupported operation: Cannot add to an unmodifiable list
      chat.messages.add(message);
  
      notifyListeners();
    }
  </code></pre>