Extend/Override core block controller in package

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Hey,

I need to override/extend a core block type controller in my package so I can extend its methods. My current use case is to use registerViewAssets() in my extended controller to require assets for a custom template provided in my package. Here's what I have so far in my controller.php located in packages\testpkg\blocks\image_slider\
<?php
namespace Concrete\Package\Testpkg\Block\ImageSlider;
use Concrete\Block\ImageSlider\Controller as ImageSliderBlockController; // InheritedBlockController
class Controller extends ImageSliderBlockController {
   public function registerViewAssets() {
      $this->requireAsset('bxslider'); // bxslider is an asset group registered in my package on_start()
      echo 'registerViewAssets() in package block override!'; // above requireAsset() doesnt include assets in page
   }
}