title: Custom blocks not working in Drupal 8
body: <p>I'm building a module with custom block in Drupal 8 beta 9. When I create a block in my module I'm placing it in src\Plugin\Block directory. The block show in 'Block structure' list, but when I press link to add it, nothing happens.
  I reviewed my code 10 times, tried to put my files into core module's directories (only for test obviously), tried to put some core block files to my directory, to copy-paste code. None of this works. Here is a code of my block file: </p>
  
  <pre><code>&lt;?php
  /**
   * @file
   * Contains \Drupal\internetdevels\Plugin\Block\TestBlock.
   */
  
  namespace Drupal\internetdevels\Plugin\Block;
  use Drupal\Core\Block\BlockBase;
  
  /**
   * Provides 'my custom' block.
   *
   * @Block(
   *   id = "my_custom_block",
   *   admin_label = @Translation("My Custom Block"),
   *   category = @Translation("System"),
   * )
   */
  class TestBlock extends BlockBase {
  
    /**
     * {@inheritdoc}
     */
    public function build() {
      return array('#markup' =&gt; 'hello world');
    }
  
  }
  </code></pre>