Uncaught ReferenceError: _asyncToGenerator is not defined

I'm seeing the following error when building my app with `ember-cli-babel@6.5.0`:

```
Uncaught ReferenceError: _asyncToGenerator is not defined
```

The source file contains:

```
import { assert } from '@ember/debug';
```

and once I remove that line the `_asyncToGenerator()` is correctly included into the output.