Mark a future task instance to success including downstream tasks using CLI

Assume, there is a DAG with ID a_dag and five tasks with IDs dummy1, dummy2, dummy3, dummy4, dummy5. We want to mark the dummy3 task and all it's downstream tasks - dummy4,dummy5 etc to success from the next DAG run. Since it's a future task instance, we can't do that in the UI. So we can use make use of CLI.

If we want to mark a future task instance to success, we can use below command.
airflow run -fAIim a_dag dummy3 2020-07-11T20:00:00+00:00

It would be great if we include downstream tasks in the above command, so that we do not have to run the above command for each downstream tasks separately.

Start by locating the implementation of the `airflow run` CLI command and how it handles the `-fAIim` options for future task instances. Trace how downstream tasks are identified, then define and test the behavior for marking dummy3 and all downstream tasks successful in the specified future DAG run.

Written by the indexing model from the issue text.