ES#sec-advancestringindex AdvanceStringIndex ( S, index, unicode )
function AdvanceStringIndex(string, index, unicode) {
  var increment = 1;
  if (unicode) {
    var first = %_StringCharCodeAt(string, index);
    if (first >= 0xD800 && first <= 0xDBFF && string.length > index + 1) {
      var second = %_StringCharCodeAt(string, index + 1);
      if (second >= 0xDC00 && second <= 0xDFFF) {
        increment = 2;
      }
    }
  }
  return increment;
}