anchor.diff:        }
       }
       this._imports = imports;
  +    this._prefixes = null;
     }
   
     @override
  
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
  