setOverlay method
Overlay the content of the file at the given path. The file will appear
to have the given content and modificationStamp even if the file is
modified in the base resource provider.
Implementation
void setOverlay(
  String path, {
  required String content,
  required int modificationStamp,
}) {
  _overlays[path] = _OverlayFileData(content, modificationStamp);
}