close method
Closes the internal StreamController
Implementation
@override
void close() {
  if (_closed) return;
  _closed = true;
  _future.flatMap((_) => _controller?.close());
}Closes the internal StreamController
@override
void close() {
  if (_closed) return;
  _closed = true;
  _future.flatMap((_) => _controller?.close());
}