**Apache Airflow version**:

master

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

**Environment**: breeze

**What happened**:

This DAG
```python
from airflow import models
from airflow.operators.dummy_operator import DummyOperator
from airflow.utils.dates import days_ago

with models.DAG("test", start_date=days_ago(1), schedule_interval=None,) as dag:
    t1 = DummyOperator(task_id="t1")
    t2 = DummyOperator(task_id="t2")
    t1 >> t2
```

is rendering like that:

<img width="1374" alt="Screenshot 2020-08-27 at 19 59 41" src="https://user-images.githubusercontent.com/9528307/91478403-11d7fb00-e8a0-11ea-91d0-d7d578bcb5a2.png">

**What you expected to happen**:

I expect to see same relations as defined in DAG file

**How to reproduce it**:

Render the example DAG from above.


**Anything else we need to know**:

I'm surprised by this bug 👀 

airflow/www/views.py
nonlocal node_count
            node_count += 1
            visited.add(task)
            task_id = task.task_id
            node = {
                'name': task.task_id,
                'instances': [
                    encode_ti(task_instances.get((task_id, d)))
                    for d in dates
                ],
                'num_dep': len(task.downstream_list),
                'operator': task.task_type,
                'retries': task.retries,
                'owner': task.owner,
                'ui_color': task.ui_color,
            }
            if task.downstream_list:
                children = [
                    recurse_nodes(t, visited) for t in task.downstream_list
                    if node_count < node_limit or t not in visited]
                
                
                
                if task.task_id not in expanded:
                    children_key = 'children'
                    expanded.add(task.task_id)
                else:
                    children_key = "_children"