addStream method Null safety
Adds all elements of the given
Returns a Future that completes when
all elements of the given
stream have been added.
If the stream contains an error, the
addStream ends at the error,
and the returned future completes with that error.
This function must not be called when a stream is currently being added using this function.
Future addStream(Stream<List<int>> stream) => _sink.addStream(stream);