store method
Stores bytes under key, creating root when needed.
Throws an ArgumentError when key is not 16 lower-case hex characters
(the shape frameKey produces).
Implementation
Future<void> store(String key, Uint8List bytes) async {
_checkKey(key);
await root.create(recursive: true);
await File('${root.path}/$key').writeAsBytes(bytes, flush: true);
}