dispose method

Implementation

@override
void dispose() {
  disconnect();

  for (final plugin in plugins) {
    plugin.dispose();
  }

  _messageController.close();
  _connectionStatusController.close();
  // Not awaited: `dispose` is synchronous, and the flag inside is set before
  // the first await, so nothing more reaches this client either way.
  unawaited(_syncManager.dispose());
}