Issue #2567571 follow-up by mikeryan, benjy: Small simplification to code.

core/modules/migrate/src/Plugin/migrate/source/SourcePluginBase.php

@@ -192,7 +192,7 @@ public function prepareRow(Row $row) {
       // Make sure we replace any previous messages for this item with any
       // new ones.
       if ($save_to_map) {
-        $this->migration->getIdMap()->saveIdMapping($row, array(), MigrateIdMapInterface::STATUS_IGNORED);
+        $this->idMap->saveIdMapping($row, array(), MigrateIdMapInterface::STATUS_IGNORED);
         $this->currentRow = NULL;
         $this->currentSourceIds = NULL;
       }