anchor.diff:        }
       }
       this._imports = imports;
  +    this._prefixes = null;
     }
   
     @override
  
positive.code: 
    @override
    List<PrefixElement> get prefixes {
      HashSet<PrefixElement> prefixes = new HashSet<PrefixElement>();
      for (ImportElement element in _imports) {
        PrefixElement prefix = element.prefix;
        if (prefix != null) {
          prefixes.add(prefix);
        }
      }
      return new List.from(prefixes);
    }
  
    @override
  
negative.code: 
    PrefixElement get prefix {
      if (_unlinkedImport != null) {
        if (_unlinkedImport.prefixReference != 0 && _prefix == null) {
          LibraryElementImpl library = enclosingElement as LibraryElementImpl;
          _prefix = new PrefixElementImpl.forSerialized(_unlinkedImport, library);
        }
  