cancel method
Cancels this operation.
This returns the Future returned by the CancelableCompleter's
onCancel callback. Unlike Stream.cancel, it never returns null.
Implementation
Future cancel() => _completer._cancel();Cancels this operation.
This returns the Future returned by the CancelableCompleter's
onCancel callback. Unlike Stream.cancel, it never returns null.
Future cancel() => _completer._cancel();