anchor.diff:              ? const {}
               : UnmodifiableMapView(Map.from(features)),
           super._(name, source, description);
  -
  -  PackageRange.magic(String name)
  -      : constraint = Version.none,
  -        features = const {},
  -        super._magic(name);
   
     /// Creates a range that selects the root package.
     PackageRange.root(Package package)
  
positive.code:   @override
    String toString([PackageDetail detail]) {
      detail ??= PackageDetail.defaults;
      if (isMagic) return name;
  
      var buffer = StringBuffer(name);
      if (detail.showVersion ?? !isRoot) buffer.write(' $version');
  
negative.code:   PackageId(String name, Source source, this.version, description)
        : super._(name, source, description);
  
    /// Creates an ID for the given root package.
    PackageId.root(Package package)
        : version = package.version,
  