Directory subdirectoryInProjectDirectory(String name, { bool createIfDoesNotExist: true })

Source 

Directory subdirectoryInProjectDirectory(String name,
    {bool createIfDoesNotExist: true}) {
  var dir = new Directory(name);
  if (path_lib.isRelative(name)) {
    dir = new Directory.fromUri(projectDirectory.uri.resolve(name));
  }

  if (createIfDoesNotExist && !dir.existsSync()) {
    dir.createSync();
  }

  return dir;
}