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: }
  
  abstract class PlatformFlags {
    /// Denotes a package that references Flutter in `pubspec.yaml`.
    static const String flutter = 'flutter';
  
    /// Denotes a library that depends on a native extensions via `dart-ext:`
    static const String dartExtension = 'dart-ext';
  }
  
  @JsonSerializable()
  
negative.code:   String get description {
      if (pubspec.isFlutter) {
        if (libraries.values.every((pi) => pi.worksOnFlutter)) {
          return PlatformFlags.flutter;
        }
        assert(hasConflict);
        return PlatformFlags.conflict;
      }
  
      assert(pubspec == PubspecPlatform.undefined);
  