writeAtSync method
Same-isolate commit path. The pager calls these in one straight-line sequence so a file statement does not pay an event-loop turn per syscall.
Implementation
@override
void writeAtSync(int offset, Uint8List bytes) {
if (_closed) throw StateError('Storage closed');
if (offset < 0) throw RangeError.value(offset);
_file.setPositionSync(offset);
_file.writeFromSync(bytes);
}