docs: public static void appendHexEscape(StringBuilder out, int codePoint) {
  
      if (Character.isSupplementaryCodePoint(codePoint)) {
        // Handle supplementary unicode values which are not representable in
        // javascript.  We deal with these by escaping them as two 4B sequences
        // so that they will round-trip properly when sent from java to javascript
        // and back.
        char[] surrogates = Character.toChars(codePoint);
        appendHexEscape(out, surrogates[0]);
        appendHexEscape(out, surrogates[1]);
  
      } else {
        out.append("\\u")
            .append(HEX_DIGITS[(codePoint >>> 12) & 0xF])
            .append(HEX_DIGITS[(codePoint >>> 8) & 0xF])
            .append(HEX_DIGITS[(codePoint >>> 4) & 0xF])
            .append(HEX_DIGITS[codePoint & 0xF]);
      }
    }