[Feature] Add generics support to AbstractControl.get method

[desfero]

I'm submitting a ... (check one with "x")

[ ] bug report => search github for a similar issue or PR before submitting
[x ] feature request
[ ] support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

What is the motivation / use case for changing the behavior?
To get custom controls without typescript assertions.

Can be implemented as follow (assuming TypeScript 2.3 'Generic parameter defaults')

get<T extends AbstractControl = AbstractControl>(path: Array<string|number>|string): T|null { ... }

Current behavior

class CustomFormControl extends FormControl {
  doSomething() ...
}

const formGroup = new FormGroup({
    foo: new CustomFormContro(),
})

const fooControl: CustomFormControl = formGroup.get('foo') as CustomFormControl;
fooControl.doSomething();

Featured behavior

class CustomFormControl extends FormControl {
  doSomething() ...
}

const formGroup = new FormGroup({
    foo: new CustomFormContro(),
})

const fooControl= formGroup.get<CustomFormControl>('foo');
fooControl.doSomething();

If that make sense let me know and i will submit a PR

[Simon-Briggs]

related?: #13721 (comment)

[desfero]

Yes, you are right. Thanks

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}