3510925: Allow user to duplicate a page.

Closes #3510925

phpstan-stubs/EntityTypeManagerInterface.stub

+<?php
+
+namespace Drupal\Core\Entity;
+
+use Drupal\Core\Entity\Exception\InvalidPluginDefinitionException;
+
+/**
+ * Stub for conditional return, it throws an exception instead of returning null.
+ * @todo Remove once https://github.com/mglaman/phpstan-drupal/issues/859 is merged.
+ *
+ * @method EntityTypeInterface getDefinition(string $entity_type_id, bool $exception_on_invalid = TRUE) throws InvalidPluginDefinitionException
+ */
+interface EntityTypeManagerInterface {}
\ No newline at end of file