Because
- `Form` is **exported** -- see line 30/31 of
`modules/@angular/forms/src/forms.ts`: i.e., <br>`export {Form} from
'./directives/form_interface'`; and
- Methods of `Form`, which are public, have an
`AbstractFormGroupDirective` parameter;
e.g.,<br>`Form.getFormGroup(dir: AbstractFormGroupDirective):
FormGroup`.
Then it makes sense for `AbstractFormGroupDirective` to be
public/exported too. In any case, if it isn't exported then the **API
docs for `Form` don't get generated properly.**
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