**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
"""
        This test verifies that there are no new fields added to BaseOperator. And reminds that
        tests should be added for it.
        """
        base_operator = BaseOperator(task_id="10")
        fields = base_operator.__dict__
        assert {
            '_BaseOperator__instantiated': True,
            '_dag': None,
            '_downstream_task_ids': set(),
            '_inlets': [],
            '_log': base_operator.log,
            '_outlets': [],
            '_upstream_task_ids': set(),
            'depends_on_past': False,
            'do_xcom_push': True,
            'email': None,
            'email_on_failure': True,
            'email_on_retry': True,
            'end_date': None,
            'execution_timeout': None,
            'executor_config': {},
            'inlets': [],
            'label': '10',
            'max_retry_delay': None,
            'on_execute_callback': None,
            'on_failure_callback': None,
            'on_retry_callback': None,
            'on_success_callback': None,