merging streams

gulpfile.js

// Sass and css libs
gulp.task('css', function () {
  var sassStream = gulp.src('./assets/*.scss').pipe(sass());
  var cssFiles = gulp.src('./assets/csslib/*.css');

  var queue = new StreamQueue();
  queue.queue(
    cssFiles,
    sassStream
  );
  queue.done();

  queue.pipe(concat('comp.css'))
    .pipe(gulp.dest('./public/css'));
});

// Sass and css libs
gulp.task('css', function () {
  var sassStream = gulp.src('./assets/*.scss').pipe(sass());
  var cssFiles = gulp.src('./assets/csslib/*.css');
  new StreamQueue({objectMode: true},
    cssFiles,
    sassStream
  ).pipe(concat('comp.css'))
    .pipe(gulp.dest('./public/css'));
});