anchor.diff:     * A bit-encoded form of the capabilities associated with this library.
      */
     int _resolutionCapabilities = 0;
  +
  +  /**
  +   * The cached list of prefixes.
  +   */
  +  List<PrefixElement> _prefixes;
   
     /**
      * Initialize a newly created library element in the given [context] to have
  
positive.code: 
    PrefixElement get prefix {
      if (_unlinkedImport != null) {
        if (_unlinkedImport.prefixReference != 0) {
          LibraryElementImpl library = enclosingElement as LibraryElementImpl;
          _prefix = new PrefixElementImpl.forSerialized(_unlinkedImport, library);
        }
  
negative.code: 
    @override
    List<PrefixElement> get prefixes {
      if (_prefixes == null) {
        HashSet<PrefixElement> prefixes = new HashSet<PrefixElement>();
        for (ImportElement element in _imports) {
          PrefixElement prefix = element.prefix;
          if (prefix != null) {
            prefixes.add(prefix);
          }
        }
        _prefixes = prefixes.toList();
      }
      return _prefixes;
    }
  
    @override
  