done method Null safety
Finalize and return the written ByteData.
Implementation
ByteData done() {
if (_isDone) {
throw StateError('done() must not be called more than once on the same $runtimeType.');
}
final ByteData result = _buffer.buffer.asByteData(0, _currentSize);
_buffer = Uint8List(0);
_isDone = true;
return result;
}