State enum for 'aria-pressed' values.

class PressedValue<int> extends Enum<int> implements AriaAttributeType {

 //, FALSE, MIXED, UNDEFINED;
 static const PressedValue TRUE = const PressedValue(0);
 static const PressedValue FALSE = const PressedValue(1);
 static const PressedValue MIXED = const PressedValue(2);
 static const PressedValue UNDEFINED = const PressedValue(3);

 const PressedValue(int type) : super (type);

 /**
  * Gets the enum constant corresponding to {@code value} for the token type
  * PressedValue.
  *
  * @param value Boolean value for which we want to get the corresponding enum constant.
  */
 static PressedValue of(bool value) {
   return value ? TRUE : FALSE;
 }

 String getAriaValue() {
   switch (this) {
       case TRUE:
         return "true";
       case FALSE:
         return "false";
       case MIXED:
         return "mixed";
       case UNDEFINED:
         return "undefined";
   }
   return null; // not reachable
 }
}