anchor.diff:  
     Set<String> get descriptionSet {
       if (worksEverywhere) {
  -      return new Set.from(['everywhere']);
  +      return new Set.from([PlatformFlags.everywhere]);
       }
   
       var items = <String>[];
       if (worksOnFlutter) {
  -      items.add('flutter');
  +      items.add(PlatformFlags.flutter);
       }
   
       if (worksOnServer) {
  -      items.add('server');
  +      items.add(PlatformFlags.server);
       }
   
       if (worksOnWeb) {
  -      items.add('web');
  +      items.add(PlatformFlags.web);
       }
   
       if (items.isEmpty) {
         assert(hasConflict);
  -      return new Set.from(['conflict']);
  +      return new Set.from([PlatformFlags.conflict]);
       }
   
       assert(!hasConflict);
  
positive.code:   String get description {
      if (pubspec.isFlutter) {
        if (libraries.values.every((pi) => pi.worksOnFlutter)) {
          return 'flutter';
        }
        assert(hasConflict);
        return 'conflict';
      }
  
      assert(pubspec == PubspecPlatform.undefined);
  
negative.code: class PubspecPlatform {
    final String description;
  
    static const PubspecPlatform flutter =
        const PubspecPlatform._(PlatformFlags.flutter);
  
    static const PubspecPlatform undefined =
        const PubspecPlatform._(PlatformFlags.undefined);
  
    bool get isFlutter => description == flutter.description;
  
  