This method is used in DagFileProcessor.process_file - it may be easier to migrate all it's internal calls instead of the whole method.
def standard_subdag():
            import datetime
            from airflow.models import DAG
            from airflow.operators.empty import EmptyOperator
            from airflow.operators.subdag import SubDagOperator
            dag_name = "parent"
            default_args = {"owner": "owner1", "start_date": datetime.datetime(2016, 1, 1)}
            dag = DAG(dag_name, default_args=default_args)
            
            
            
            
            
            
            
            with dag:
                def subdag_0():