Move controller render array -> HtmlFragment conversion to a View listener; eliminate HtmlPageController

1. +++ b/core/lib/Drupal/Core/Controller/CreateHtmlFragmentTrait.php
   @@ -0,0 +1,62 @@
   +trait CreateHtmlFragmentTrait {
   +
   +  /**
   +   * The title resolver.
   +   *
   +   * @var \Drupal\Core\Controller\TitleResolverInterface
   +   */
   +  protected $titleResolver;
   +
   +  /**
   +   * The render array to HTML fragment renderer.
   +   *
   +   * @var \Drupal\Core\Page\RenderHtmlRendererInterface
   +   */
   +  protected $renderHtmlRenderer;
   +
   

   I'm unclear on the point of this trait.

   If making a trait like this, don't rely on properties being defined properly. You can't guarantee what the using class will do. Instead, get these services via a protected abstract method. Then it's the using class's responsibility to get access to those services and expose them via those methods.

2. +++ b/core/lib/Drupal/Core/Controller/CreateHtmlFragmentTrait.php
   @@ -0,0 +1,62 @@
   +    // Allow controllers to return a HtmlFragment or a Response object directly.
   

   This trait doesn't technically no from controllers. Just "If the content is already a fragment or a response there is no work to do."

3. +++ b/core/lib/Drupal/Core/EventSubscriber/HtmlViewSubscriber.php
   @@ -42,10 +46,31 @@ class HtmlViewSubscriber implements EventSubscriberInterface {
   +    if (is_array($result)) {
   +      $renderer = $this->htmlRenderer;
   +      $fragment = $this->createHtmlFragment($result, $event->getRequest());
   +      $event->setControllerResult($fragment);
   +    }
   

   The $renderer = line seems unused.

It looks like a lot of the test fails are fatal errors, which means they should be easy to replicate manually in the UI for debugging.