anchor.diff:          migration, includedRoot, listener, instrumentationListener);
       await state.refresh();
   
  -    if (_server == null) {
  +    if (enablePreview && _server == null) {
         _server = HttpPreviewServer(state, rerun, preferredPort);
         _server.serveHttp();
         port = await _server.boundPort;
  
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:   /// 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;
  
  