onClose method
Closes the connection, naming the protocol's code and reason.
The default closes the channel and loses the code, because a
StreamChannel has nowhere to put it. Override this on a WebSocket
transport and close the socket with code: it is how a client tells a
refused connection from a malformed message from a duplicated id.
Implementation
FutureOr<void> onClose(int code, String reason) {
client.close();
}