function getAstralNumberFromSurrogatePair(highSurrogate, lowSurrogate) {
    return (highSurrogate - HIGH_SURROGATE_START) * 0x400 + lowSurrogate - LOW_SURROGATE_START + 0x10000;
  }
Get the astral code point number based on surrogate pair numbers.

@ignore
@param {number} highSurrogate The high-surrogate code point number.
@param {number} lowSurrogate The low-surrogate code point number.
@return {number} Returns the astral symbol number.