Seal FileSystemEntity.

[modulovalue]

Similar to #52027 and #51912, could FileSystemEntity also be sealed so that functions like:

Z matchFileSystemEntity<Z>({
  required final FileSystemEntity entity,
  required final Z Function(Directory) directory,
  required final Z Function(File) file,
  required final Z Function(Link) link,
}) {
  if (entity is Directory) {
    return directory(entity);
  } else if (entity is File) {
    return file(entity);
  } else if (entity is Link) {
    return link(entity);
  } else {
    throw Exception(
      "Invalid state. " + entity.toString() + " was expected to be a directory, file or link.",
    );
  }
}

can be replaced with exhaustive switches?

[modulovalue]

FileSystemEntityType.notFound seems to imply that sealing FileSystemEntity in a way where it only has 3 variants (File, Directory & Link) is not possible and would require a fourth variant e.g. "UnknownFileSystemEntity"?

[sortie]

Note how there's a number of file types that exist but that Dart doesn't really support, such as block devices and character devices. It's a bad idea to write code that exhaustively consider file types.

[brianquinlan]

This isn't really practical because there is a lot of code that extends or implements FileSystemEntity (or it's subclasses) for testing purposes.