code: public static CalendarSystem forName(String calendarName) {
          if ("gregorian".equals(calendarName)) {
              return GregorianHolder.INSTANCE;
          } else if ("julian".equals(calendarName)) {
              return JulianHolder.INSTANCE;
          }
          return null;
  
          /* J2ObjC changed: Only "gregorian" and "julian" calendars are supported.
          CalendarSystem cal = calendars.get(calendarName);
          if (cal != null) {
              return cal;
          }
  
          Class<?> calendarClass = names.get(calendarName);
          if (calendarClass == null) {
              return null; // Unknown calendar name
          }
  
          if (calendarClass.isAssignableFrom(LocalGregorianCalendar.class)) {
              // Create the specific kind of local Gregorian calendar system
              cal = LocalGregorianCalendar.getLocalGregorianCalendar(calendarName);
          } else {
              try {
                  cal = (CalendarSystem) calendarClass.newInstance();
              } catch (Exception e) {
                  throw new RuntimeException("internal error", e);
              }
          }
          if (cal == null) {
              return null;
          }
  
          CalendarSystem cs =  calendars.putIfAbsent(calendarName, cal);
          return (cs == null) ? cal : cs;*/
      }
docs: Returns a <code>CalendarSystem</code> specified by the calendar
  name. The calendar name has to be one of the supported calendar
  names.
  
  @param calendarName the calendar name
  @return the <code>CalendarSystem</code> specified by
  <code>calendarName</code>, or null if there is no
  <code>CalendarSystem</code> associated with the given calendar name.