Cannot assign to read only property 'entity' of object '[object Object]'

Hi,
I tried to use you library on version `v1.0.0-beta.2`
With the following code:
```js
import Component from '@ember/component';
import { tagName } from '@ember-decorators/component';
import { argument } from '@ember-decorators/argument';
import { Action } from '@ember-decorators/argument/types';
import Model from '@ember-data/model';

@tagName('')
export default class MyComponent extends Component {
@argument(Model) entity;
@argument(Action) save;
@argument(Action) discard;
}
```

But I keep getting the following error:
```
Cannot assign to read only property 'entity' of object '[object Object]'
```

It works perfectly fine on `v0.8.21`.

What am I doing wrong ?