title: drupal 8, how debug a class?
body: <p>I want to train myself to use Drupal 8 (Im from drupal 7 =) ). So, I learned some symfony2 basics. 
  Now, I would like know if there is a good way about the code debugging, I mean :
  Before with Drupal 7, I used the devel module for show the content of data (very usefull !). I try this module for drupal 8 but the dpm function seems not work in the class context.</p>
  
  <p>Bu example, I use this code for alter some route :</p>
  
  <pre><code>&lt;?php
  namespace...
  use...
  ...
  
  class RouteSubscriber extends RouteSubscriberBase{
    public function alterRoutes(RouteCollection $collection){
      // What is the best way for display the $collection array ? like a dpm ? 
      dpm($collection); // doen't work, nothing appears in any page
    }
  </code></pre>
  
  <p>Thanks =)</p>