title: How to send message in Flutter to a channel websocket?
body: <p>I write an app flutter connect websocket. I use IOWebsocketChanel:
  My code Flutter:</p>
  
  <pre><code>widget.channel = IOWebSocketChannel.connect('ws://10.2.2.89:8080/socket');
  </code></pre>
  
  <p>I connect success.
  My code server:</p>
  
  <pre><code>@MessageMapping("/chat/message")
  public void chatMessage(String message) {
    simpMessagingTemplate.convertAndSend("/chat", message);
  }
  </code></pre>
  
  <p>But my problem:
  How i send message from flutter app to path "/chat/message"? What IOWebsocketChanel function?
  I read docs and example. They use chanel.sink.add(message). But It not send message to path "/chat/message"
  Please help me.</p>