function indexCodePointFor(pointer, index) {
    if (!index) return null;
    return index[pointer] || null;
  }
@param {number} pointer The |pointer| to search for.
@param {(!Array.<?number>|undefined)} index The |index| to search within.
@return {?number} The code point corresponding to |pointer| in |index|,
    or null if |code point| is not in |index|.