Dirty patch for https://www.drupal.org/node/1984588

drupal8-block_content-revisions.patch

diff --git a/core/modules/block_content/block_content.info.yml b/core/modules/block_content/block_content.info.yml
index b9ae564..0dd5b56 100644
--- a/core/modules/block_content/block_content.info.yml
+++ b/core/modules/block_content/block_content.info.yml
@@ -6,6 +6,7 @@ version: VERSION
 core: 8.x
 dependencies:
   - block
+  - entity
   - text
   - user
 configure: entity.block_content.collection
diff --git a/core/modules/block_content/block_content.install b/core/modules/block_content/block_content.install
index 4279cde..db404ba 100644
--- a/core/modules/block_content/block_content.install
+++ b/core/modules/block_content/block_content.install
@@ -61,3 +61,24 @@ function block_content_update_8003() {
   \Drupal::entityDefinitionUpdateManager()
     ->installFieldStorageDefinition('revision_user', 'block_content', 'block_content', $revision_user);
 }
+
+/**
+ * Add 'revision_log_message' field to 'block_content' entities.
+ */
+function block_content_update_8004() {
+  $revision_log_message = BaseFieldDefinition::create('string_long')
+    ->setLabel(t('Revision log message'))
+    ->setDescription(t('Briefly describe the changes you have made.'))
+    ->setRevisionable(TRUE)
+    ->setDefaultValue('')
+    ->setDisplayOptions('form', [
+      'type' => 'string_textarea',
+      'weight' => 25,
+      'settings' => [
+        'rows' => 4,
+      ],
+    ]);
+
+  \Drupal::entityDefinitionUpdateManager()
+    ->installFieldStorageDefinition('revision_log_message', 'block_content', 'block_content', $revision_log_message);
+}
diff --git a/core/modules/block_content/src/Entity/BlockContent.php b/core/modules/block_content/src/Entity/BlockContent.php
index 13af342..48f7be1 100644
--- a/core/modules/block_content/src/Entity/BlockContent.php
+++ b/core/modules/block_content/src/Entity/BlockContent.php
@@ -9,6 +9,8 @@ use Drupal\Core\Entity\EntityTypeInterface;
 use Drupal\Core\Field\BaseFieldDefinition;
 use Drupal\block_content\BlockContentInterface;
 use Drupal\user\UserInterface;
+use Drupal\entity\Revision\RevisionableContentEntityBase;
+
 
 /**
  * Defines the custom block entity class.
@@ -29,6 +31,9 @@ use Drupal\user\UserInterface;
  *       "delete" = "Drupal\block_content\Form\BlockContentDeleteForm",
  *       "default" = "Drupal\block_content\BlockContentForm"
  *     },
+ *     "route_provider" = {
+ *       "revision" = "\Drupal\entity\Routing\RevisionRouteProvider",
+ *     },
  *     "translation" = "Drupal\block_content\BlockContentTranslationHandler"
  *   },
  *   admin_permission = "administer blocks",
@@ -40,6 +45,9 @@ use Drupal\user\UserInterface;
  *     "delete-form" = "/block/{block_content}/delete",
  *     "edit-form" = "/block/{block_content}",
  *     "collection" = "/admin/structure/block/block-content",
+ *     "revision" = "/block/{block_content}/revisions/{block_content_revision}/view",
+ *     "revision-revert-form" = "/block/{block_content}/revisions/{block_content_revision}/revert",
+ *     "version-history" = "/block/{block_content}/revisions",
  *   },
  *   translatable = TRUE,
  *   entity_keys = {
@@ -50,6 +58,7 @@ use Drupal\user\UserInterface;
  *     "langcode" = "langcode",
  *     "uuid" = "uuid"
  *   },
+ *
  *   bundle_entity_type = "block_content_type",
  *   field_ui_base_route = "entity.block_content_type.edit_form",
  *   render_cache = FALSE,
@@ -60,7 +69,7 @@ use Drupal\user\UserInterface;
  * caching.
  * See https://www.drupal.org/node/2284917#comment-9132521 for more information.
  */
-class BlockContent extends ContentEntityBase implements BlockContentInterface {
+class BlockContent extends RevisionableContentEntityBase implements BlockContentInterface {
 
   use EntityChangedTrait;
 
@@ -237,7 +246,8 @@ class BlockContent extends ContentEntityBase implements BlockContentInterface {
    * {@inheritdoc}
    */
   public function getRevisionCreationTime() {
-    return $this->get('revision_created')->value;
+    $timestamp = $this->get('revision_created')->value;
+    return $timestamp ?: 0;
   }
 
   /**