RoboFile.php

<?php

class RoboFile extends \Robo\Tasks
{
  public function buildAssets($input, $output) {
    $result = $this->taskExec('sassc')
      ->arg('-m')
      ->arg('-t')
      ->arg('expanded')
      ->arg('-l')
      ->arg($input)
      ->arg($output)
      ->run();

    return $result;
  }

  public function buildAssetsAll() {
    $this->buildAssets('sass/screen.scss', 'stylesheets/screen.css');
    $this->buildAssets('sass/drupal-admin.scss', 'stylesheets/drupal-admin.css');
  }

  public function watchAssets() {
    $files = iterator_to_array(\Symfony\Component\Finder\Finder::create()->name('*.scss')->notName('drupal-admin.scss')->in('sass'));
    $files[] = 'sass/screen.scss';

    $this
      ->taskWatch()
      ->monitor($files, function () {
        $this->buildAssets('sass/screen.scss', 'stylesheets/screen.css');
      })
      ->monitor(['sass/drupal-admin.scss'], function () {
        $this->buildAssets('sass/drupal-admin.scss', 'stylesheets/drupal-admin.css');
      })
      ->run();
  }
}