Collects the data of this stream in a Uint8List.
Source
Future<Uint8List> toBytes() {
Completer<Uint8List> completer = new Completer<Uint8List>();
dynamic sink = new ByteConversionSink.withCallback((dynamic bytes) =>
completer.complete(new Uint8List.fromList(bytes)));
listen(sink.add, onError: completer.completeError, onDone: sink.close,
cancelOnError: true);
return completer.future;
}