anchor.diff:  
     Future _migrateTraffic() async {
       final List<String> args = [
  -      'app',
         'services',
         'set-traffic',
         service,
  
positive.code: 
    Future _gcloudDeploy() async {
      final String yamlFile = service == 'default' ? 'app.yaml' : '$service.yaml';
      final pr = await Process.run(
        'gcloud',
        ['app', 'deploy', yamlFile, '--no-promote', '-v', newVersion, '-q'],
      );
      if (pr.exitCode != 0) {
        print('[ERR] Couldn\'t deploy $service.');
  
negative.code:   }
  
    Future _detectOldVersion() async {
      final pr = await _runGCloudApp(
        ['versions', 'list', '--service', service, '--format=value(id)'],
      );
      if (pr.exitCode != 0) {
        print('[ERR] Couldn\'t detect old $service version.');
  