// Look for the first non-ASCII character.
    for (int i = off; i < end; i++) {
      if (bytes[i] < 0) {
        return isWellFormedSlowPath(bytes, i, end);
      }
    }
    return true;
  }

  private static boolean isWellFormedSlowPath(byte[] bytes, int off, int end) {
    int index = off;
    while (true) {
      int byte1;

      // Optimize for interior runs of ASCII bytes.