code: public static void illegalArgumentIf(boolean tester, String msg, Object... args) {
          if (tester) {
              throw new IllegalArgumentException(S.fmt(msg, args));
          }
      }
docs: Throws out an {@link IllegalArgumentException} with error message specified
  if `tester` is `true`.
  
  @param tester
  when `true` then throw out the exception.
  @param msg
  the error message format pattern.
  @param args
  the error message format arguments.