anchor.diff:    /// which all included paths share.
     final String includedRoot;
   
  +  /// Indicates whether the web preview of migration results should be launched.
  +  final bool enablePreview;
  +
     /// The HTTP server that serves the preview tool.
     HttpPreviewServer _server;
   
  
positive.code:   Future<void> Function([List<String>]) rerunFunction;
  
    NonNullableFix(this.listener,
        {List<String> included = const [], this.preferredPort})
        : includedRoot =
              _getIncludedRoot(included, listener.server.resourceProvider) {
      reset();
  
negative.code:         migration, includedRoot, listener, instrumentationListener);
      await state.refresh();
  
      if (enablePreview && _server == null) {
        _server = HttpPreviewServer(state, rerun, preferredPort);
        _server.serveHttp();
        port = await _server.boundPort;
  