--- /dev/null
+<?php
+
+namespace Drupal\entity_test;
+
+use Drupal\Core\Field\BaseFieldDefinition;
+
+/**
+ * A custom field storage definition class.
+ *
+ * For convenience we extend from BaseFieldDefinition although this should not
+ * implement FieldDefinitionInterface.
+ *
+ * @todo Provide and make use of a proper FieldStorageDefinition class instead:
+ * https://www.drupal.org/node/2280639.
+ */
+class FieldStorageDefinition extends BaseFieldDefinition {
+
+ /**
+ * {@inheritdoc}
+ */
+ public function isBaseField() {
+ return FALSE;
+ }
+
+}