use yii\base\Event in BaseAR to fix phpdoc reference

framework/db/BaseActiveRecord.php

@@ -8,6 +8,7 @@
 namespace yii\db;
 
 use yii\base\InvalidConfigException;
+use yii\base\Event;
 use yii\base\Model;
 use yii\base\InvalidParamException;
 use yii\base\ModelEvent;