function positionAfterWhitespace(body, startPosition, lexer) {
  var bodyLength = body.length;
  var position = startPosition;
  while (position < bodyLength) {
    var code = charCodeAt.call(body, position);
    // tab | space | comma | BOM
    if (code === 9 || code === 32 || code === 44 || code === 0xFEFF) {
      ++position;
    } else if (code === 10) {
      // new line
      ++position;
      ++lexer.line;
      lexer.lineStart = position;
    } else if (code === 13) {
      // carriage return
      if (charCodeAt.call(body, position + 1) === 10) {
        position += 2;
      } else {
        ++position;
      }
      ++lexer.line;
      lexer.lineStart = position;
    } else {
      break;
    }
  }
  return position;
}
Reads from body starting at startPosition until it finds a non-whitespace
or commented character, then returns the position of that character for
lexing.