**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

I have the following DAG:

```py
from datetime import datetime, timedelta

from airflow import DAG
from airflow.operators.bash_operator import BashOperator

with DAG(
    dag_id="sla_trigger",
    schedule_interval="* * * * *",
    start_date=datetime(2020, 2, 3),
) as dag:
    BashOperator(
        task_id="bash_task",
        bash_command="sleep 30",
        sla=timedelta(seconds=2),
    )

```

And in my understanding this dag should result in SLA miss every time it is triggered (every minute). However, after few minutes of running I don't see any SLA misses...

**What you expected to happen**:

I expect to see SLA if task takes longer than expected.

**How to reproduce it**:

Use the dag from above.

**Anything else we need to know**:

N/A

tests/serialization/test_dag_serialization.py
'outlets': [],
            'owner': 'airflow',
            'params': {},
            'pool': 'default_pool',
            'pool_slots': 1,
            'priority_weight': 1,
            'queue': 'default',
            'resources': None,
            'retries': 0,
            'retry_delay': timedelta(0, 300),
            'retry_exponential_backoff': False,
            'run_as_user': None,
            'sla': None,
            'start_date': None,
            'subdag': None,
            'task_concurrency': None,
            'task_id': '10',
            'trigger_rule': 'all_success',
            'wait_for_downstream': False,
            'weight_rule': 'downstream',
        } == fields, """
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
     ACTION NEEDED! PLEASE READ THIS CAREFULLY AND CORRECT TESTS CAREFULLY
 Some fields were added to the BaseOperator! Please add them to the list above and make sure that
 you add support for DAG serialization - you should add the field to
 `airflow/serialization/schema.json` - they should have correct type defined there.
 Note that we do not support versioning yet so you should only add optional fields to BaseOperator.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
                         """
    def test_task_group_serialization(self):