anchor.diff:        refs.add(_normalizeLibRef(le.librarySource.uri, package, packageDir));
       });
       refs.remove('dart:core');
  -    return refs.toList();
  +    return new List<String>.unmodifiable(refs);
     }
   
     LibraryElement _getLibraryElement(String path) {
  
positive.code:   final String _packagePath;
    final UriResolver _packageResolver;
    final AnalysisContext _context;
    final _cachedLibs = <String, List<String>>{};
  
    LibraryScanner._(this.packageName, this._packagePath, this._packageResolver,
        this._context);
  
negative.code:     var fullPath = p.join(packageDir, relativePath);
      var lib = _getLibraryElement(fullPath);
      if (lib == null) return [];
      var refs = new SplayTreeSet<String>();
      lib.importedLibraries.forEach((le) {
        refs.add(_normalizeLibRef(le.librarySource.uri, package, packageDir));
      });
  