Wait for the next stream value to be available.
It is not allowed to call this function again until the future has
completed. If the returned future completes with anything except true,
the iterator is done, and no new value will ever be available.
The future may complete with an error, if the stream produces an error.
Source
/**
* Wait for the next stream value to be available.
*
* It is not allowed to call this function again until the future has
* completed. If the returned future completes with anything except `true`,
* the iterator is done, and no new value will ever be available.
*
* The future may complete with an error, if the stream produces an error.
*/
Future<bool> moveNext();