append method
Adds the given future
to queue. Note that the previous
value will be
null if the last operation caused an exception.
Implementation
Future<T> append(Future<T> Function(T? previous) future,
{Duration? timeLimit}) {
return _future = _future.then(
(previous) => _nextValue(future, previous, timeLimit: timeLimit),
onError: (_) => _nextValue(future, null, timeLimit: timeLimit),
);
}