protected static function mergeKeyed($file1_contents, $file2_contents, $exclude_keys = []) {
    // Merge keyed arrays objects.
    $merge_keys = [
      'config',
      'extra',
    ];
    $output = $file1_contents;
    foreach ($merge_keys as $key) {
      // Set empty keys to empty placeholder arrays.
      if (!array_key_exists($key, $file1_contents)) {
        $file1_contents[$key] = [];
      }
      if (!array_key_exists($key, $file2_contents)) {
        $file2_contents[$key] = [];
      }

      // Merge!
      $output[$key] = ArrayManipulator::arrayMergeRecursiveDistinct($file1_contents[$key], $file2_contents[$key]);
    }

    return $output;
  }