function readName(source, position, line, col, prev) {
  var body = source.body;
  var bodyLength = body.length;
  var end = position + 1;
  var code = 0;
  while (end !== bodyLength && (code = charCodeAt.call(body, end)) !== null && (code === 95 || // _
  code >= 48 && code <= 57 || // 0-9
  code >= 65 && code <= 90 || // A-Z
  code >= 97 && code <= 122 // a-z
  )) {
    ++end;
  }
  return new Tok(NAME, position, end, line, col, prev, slice.call(body, position, end));
}
Reads an alphanumeric + underscore name from the source.

[_A-Za-z][_0-9A-Za-z]*