Performs the coalesce.
array An array of file resources
private function getFileResources() {
  $paths = array();
  foreach ($this->directories as $directory) {
    foreach ($directory as $file) {
      $relative = $this
        ->getRelativeName($file, $directory);
      if (!isset($paths[$relative])) {
        $paths[$relative] = $file;
      }
    }
  }
  return array_values($paths);
}