function commentWhitespace() {
  let ch;
  do {
    ch = source.charCodeAt(pos);
    if (ch === 47 /* / */) {
      const nextCh = source.charCodeAt(pos + 1);
      if (nextCh === 47 /* / */) lineComment();
      else if (nextCh === 42 /***/) blockComment();
      else return ch;
    } else if (!isBrOrWs(ch)) {
      return ch;
    }
  } while (pos++ < end);
  return ch;
}
Produces the next character that is not whitespace and does not participate
in a comment, or undefined if it reaches the end of the file.