Confusing null promotion failure with while loop

main.dart

import 'package:file/file.dart';

String? findProjectRoot(FileSystem fileSystem, [ String? directory ]) {
  const String kProjectRootSentinel = 'pubspec.yaml';
  directory ??= fileSystem.currentDirectory.path;
  while (true) {
    if (fileSystem.isFileSync(fileSystem.path.join(directory, kProjectRootSentinel))) {
      return directory;
    }
    final String parent = fileSystem.path.dirname(directory);
    if (directory == parent) {
      return null;
    }
    directory = parent;
  }
}

String? findProjectRoot3(FileSystem fileSystem, [ String? directory ]) {
  const String kProjectRootSentinel = 'pubspec.yaml';
  var localDirectory = directory ?? fileSystem.currentDirectory.path;
  while (true) {
    if (fileSystem.isFileSync(fileSystem.path.join(localDirectory, kProjectRootSentinel))) {
      return localDirectory;
    }
    final String parent = fileSystem.path.dirname(localDirectory);
    if (localDirectory == parent) {
      return null;
    }
    localDirectory = parent;
  }
}


String? findProjectRoot2(FileSystem fileSystem, [ String? directory ]) {
  const String kProjectRootSentinel = 'pubspec.yaml';
  directory ??= fileSystem.currentDirectory.path;
  if (fileSystem.isFileSync(fileSystem.path.join(directory, kProjectRootSentinel))) {
    return directory;
  }
  final String parent = fileSystem.path.dirname(directory);
  if (directory == parent) {
    return null;
  }
  directory = parent;
}