title: How to use URL:: or asset() in config files in laravel 5
body: <p>In any config file If I use URL class I get the error 'Class URL not found'; if I use the function "asset", when I update composer.json I get this error:
  Catchable fatal error: Argument 2 passed to Illuminate\Routing\UrlGenerator::__construct() must be an instance of Illuminate\Http\Request, null given,</p>
  
  <p>Outside of config files both work fine</p>
  
  <pre><code>return [
      'photos_url' =&gt; URL::asset('xxx'),
  ];
  </code></pre>
  
  <p>or</p>
  
  <pre><code>return [
      'photos_url' =&gt; asset('xxx'),
  ];
  </code></pre>
  
  <p>Test</p>
  
  <pre><code>echo config('site.photos_url'); // or echo Config::get('site.photos_url');
  </code></pre>