anchor.diff:            enablePreview: options.webPreview,
             summaryPath: options.summary);
         fixCodeProcessor.registerCodeTask(nonNullableFix);
  +
         try {
           await fixCodeProcessor.runFirstPhase();
  +        fixCodeProcessor._progressBar.clear();
           _checkForErrors();
         } on StateError catch (e) {
           logger.stdout(e.toString());
  
positive.code:   Future<List<String>> runLaterPhases() async {
      for (var phase = 1; phase < numPhases; phase++) {
        await processResources((ResolvedUnitResult result) async {
          await processCodeTasks(phase, result);
        });
      }
      await finishCodeTasks();
  
      return nonNullableFixTask.previewUrls;
    }
  
negative.code: 
    final Set<String> pathsToProcess;
  
    _ProgressBar _progressBar;
  
    final MigrationCli _migrationCli;
  
    _FixCodeProcessor(this.context, this._migrationCli)
  