No methods on RendererInterface should be static

Please review.

Updated patch to pass unit test cases.

+++ b/core/lib/Drupal/Core/Ajax/AjaxResponse.php
@@ -78,7 +78,7 @@ public function addCommand(CommandInterface $command, $prepend = FALSE) {
-      $attachments = Renderer::mergeAttachments($this->attachments, $attachments);
+      $attachments = NestedArray::mergeDeep($this->attachments, $attachments);
       $this->setAttachments($attachments);

What is the reason we can use NestedArray::mergeDeep instead of the implementation of the renderer.

NestedArray::mergeDeep does exactly what static method of mergeAttachments did.

rendered service is not valid when called out from the global services container. If implementation of the renderer is to be used then (I guess) service should be injected somehow via setter methods.

Please advise / help which way to go from here?

  public function addCommand(CommandInterface $command, $prepend = FALSE) {
    if ($prepend) {
      array_unshift($this->commands, $command->render());
    }
    else {
      $this->commands[] = $command->render();
    }
    if ($command instanceof CommandWithAttachedAssetsInterface) {
      $assets = $command->getAttachedAssets();
      $attachments = [
        'library' => $assets->getLibraries(),
        'drupalSettings' => $assets->getSettings(),
      ];
      $attachments = NestedArray::mergeDeep($this->attachments, $attachments);
      $this->setAttachments($attachments);
    }

    return $this;
  }

Did try that already but unit test cases fails due to missing (global services) container.

"\Drupal::$container is not initialized yet. \Drupal::setContainer() must be called with a real container."

I can do something like this?

Then let me try with #7.

First trial to fix the last few failures.

For some reason if container created using RendererInterface (instead of Renderer), e.g.

    $this->renderer = $this->getMock('Drupal\Core\Render\RendererInterface');
    $container = new ContainerBuilder();
    $container->set('renderer', $this->renderer);
    \Drupal::setContainer($container);

returned error (even though arguments were exactly the same in mergeAttachments())
Argument 1 passed to Drupal\Core\Ajax\AjaxResponse::setAttachments() must be of the type array, null given

@Wim Leers: Thank You! Was not aware of that (until now ;-).

Updated patch, as in #17.

Thanks for the help and good advices once again!

Updated patch, changes as in #19.

This issue addresses a major bug and is allowed per https://www.drupal.org/core/beta-changes. Committed a0078c5 and pushed to 8.0.x. Thanks!

I considered asking for a CR but I don't usages of these methods with be widespread in custom or contrib yet - so I don't think it is worth the noise.