code: private static String unsignedIntToString(int x, int radix)
      {
          long asLong = x & INT_MASK;
          return Long.toString(asLong, radix);
      }
docs: Returns a string representation of {@code x} for the given radix, where {@code x} is treated as unsigned.
  
  @param x
  the value to convert to a string.
  @param radix
  the radix to use while working with {@code x}
  @throws IllegalArgumentException
  if {@code radix} is not between {@link Character#MIN_RADIX} and {@link Character#MAX_RADIX}.