anchor.diff:        message != null ? message : 'letter expected');
   }
   
  -class _LetterCharPredicate extends CharacterPredicate {
  +class _LetterCharPredicate implements CharacterPredicate {
   
     const _LetterCharPredicate();
   
  
positive.code:     return false;
    }
  
    @override
    CharacterPredicate or(CharacterPredicate other) {
      return new _AltCharacterPredicate(new List()..addAll(_predicates)..add(other));
    }
  
  }
  
  int _toCharCode(element) {
  
negative.code:       message != null ? message : 'lowercase letter expected');
  }
  
  class _LowercaseCharPredicate implements CharacterPredicate {
  
    const _LowercaseCharPredicate();
  
  