Cleanup BlockContentBlock

Thanks, but this isn't what I was chasing

+++ b/core/modules/block_content/src/Plugin/Block/BlockContentBlock.php
@@ -149,14 +149,14 @@ public function blockSubmit($form, FormStateInterface $form_state) {
-    if ($block = entity_load_by_uuid('block_content', $uuid)) {
-      return entity_view($block, $this->configuration['view_mode']);
+    if ($block = \Drupal::entityManager()->loadEntityByUuid('block_content', $uuid)) {
+      return \Drupal::entityManager()->getViewBuilder($block)->view($block, $this->configuration['view_mode']);
...
-          '!url' => url('block/add')
+          '!url' => \Drupal::urlGenerator()->generate('block_content.add_page'),

using \Drupal is no better than using procedural functions. We need to implement ContainerFactoryPluginInterface and inject the dependencies - updated the OP to make it explicit.

+++ b/core/modules/block_content/src/Plugin/Block/BlockContentBlock.php
@@ -149,14 +149,14 @@ public function blockSubmit($form, FormStateInterface $form_state) {
+      return \Drupal::entityManager()->getViewBuilder($block)->view($block, $this->configuration['view_mode']);

this expects 'block_content' not $block for getViewBuilder - thats why the tests fail

Thanks @rpayanm - awesome work!
No need to apologize.