/**
  * Constructor
  *
  * @param Filesystem $filesystem
  * @param \Magento\Framework\View\Design\Fallback\RulePool $rulePool
  * @param array $alternativeExtensions
  */
 public function __construct(Filesystem $filesystem, \Magento\Framework\View\Design\Fallback\RulePool $rulePool, array $alternativeExtensions = [])
 {
     foreach ($alternativeExtensions as $extension => $newExtensions) {
         if (!is_string($extension) || !is_array($newExtensions)) {
             throw new \InvalidArgumentException("\$alternativeExtensions must be an array with format: " . "array('ext1' => array('ext1', 'ext2'), 'ext3' => array(...)]");
         }
     }
     $this->alternativeExtensions = $alternativeExtensions;
     parent::__construct($filesystem, $rulePool);
 }