function readDigits(source, start, firstCode) {
  var body = source.body;
  var position = start;
  var code = firstCode;
  if (code >= 48 && code <= 57) {
    // 0 - 9
    do {
      code = charCodeAt.call(body, ++position);
    } while (code >= 48 && code <= 57); // 0 - 9
    return position;
  }
  throw (0, _error.syntaxError)(source, position, 'Invalid number, expected digit but got: ' + printCharCode(code) + '.');
}
Returns the new position in the source after reading digits.