appendBytes method
Append data to path (creates if absent).
Implementation
@override
Future<void> appendBytes(String path, Uint8List data) async {
final cur = _store[path] ?? Uint8List(0);
final next = Uint8List(cur.length + data.length);
next.setAll(0, cur);
next.setAll(cur.length, data);
_store[path] = next;
}