diff --git a/core/lib/Drupal/Core/Entity/EntityType.php b/core/lib/Drupal/Core/Entity/EntityType.php
index 140ca2b..0c8ca7a 100644
--- a/core/lib/Drupal/Core/Entity/EntityType.php
+++ b/core/lib/Drupal/Core/Entity/EntityType.php
@@ -233,6 +233,13 @@ class EntityType implements EntityTypeInterface {
   protected $constraints = array();
 
   /**
+   * Any additional properties and values.
+   *
+   * @var array
+   */
+  protected $additional = [];
+
+  /**
    * Constructs a new EntityType.
    *
    * @param array $definition
@@ -248,10 +255,7 @@ public function __construct($definition) {
     }
 
     foreach ($definition as $property => $value) {
-      if (!property_exists($this, $property)) {
-        throw new \InvalidArgumentException(sprintf('"%s" property does not exist on %s. Change the entity annotation for "%s" or extend this class.', $property, get_class($this), $definition['id']));
-      }
-      $this->{$property} = $value;
+      $this->set($property, $value);
     }
 
     // Ensure defaults.
@@ -282,14 +286,25 @@ public function __construct($definition) {
    * {@inheritdoc}
    */
   public function get($property) {
-    return isset($this->{$property}) ? $this->{$property} : NULL;
+    if (property_exists($this, $property)) {
+      $value = isset($this->{$property}) ? $this->{$property} : NULL;
+    }
+    else {
+      $value = isset($this->additional[$property]) ? $this->additional[$property] : NULL;
+    }
+    return $value;
   }
 
   /**
    * {@inheritdoc}
    */
   public function set($property, $value) {
-    $this->{$property} = $value;
+    if (property_exists($this, $property)) {
+      $this->{$property} = $value;
+    }
+    else {
+      $this->additional[$property] = $value;
+    }
     return $this;
   }
 
diff --git a/core/modules/migrate/src/Entity/Migration.php b/core/modules/migrate/src/Entity/Migration.php
index 262c21a..7414980 100644
--- a/core/modules/migrate/src/Entity/Migration.php
+++ b/core/modules/migrate/src/Entity/Migration.php
@@ -24,7 +24,6 @@
  * @ConfigEntityType(
  *   id = "migration",
  *   label = @Translation("Migration"),
- *   provider = "migrate",
  *   handlers = {
  *     "storage" = "Drupal\migrate\MigrationStorage"
  *   },