fix(forms): export AbstractFormGroupDirective

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.**

modules/@angular/forms/src/forms.ts

@@ -23,6 +23,7 @@
 
 export {FORM_DIRECTIVES, REACTIVE_FORM_DIRECTIVES} from './directives';
 export {AbstractControlDirective} from './directives/abstract_control_directive';
+export {AbstractFormGroupDirective} from './directives/abstract_form_group_directive';
 export {CheckboxControlValueAccessor} from './directives/checkbox_value_accessor';
 export {ControlContainer} from './directives/control_container';
 export {ControlValueAccessor, NG_VALUE_ACCESSOR} from './directives/control_value_accessor';