Memory leak when FormControlName created/destroyed few times #20007

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[x] Bug report  
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

Current behavior

1. FormControlName, objects provided by NG_VALIDATORS (may be some other) can not be collected by gc until there are refs for FormGroup.
   This is caused by collecting closures created in https://github.com/angular/angular/blob/4.4.6/packages/forms/src/directives/shared.ts#L33 in https://github.com/angular/angular/blob/4.4.6/packages/forms/src/model.ts#L744 .
2. All provided validators (via NG_VALIDATORS token) are called as many times as form is recreated.

Expected behavior

1. After destroying FormControlName it should be collected by gc even if there are refs for FormGroup.
2. Provided validators should be removed from FormControl after destroying FormControlName.

Minimal reproduction of the problem with instructions

https://plnkr.co/edit/eCtf2naoxLe8bWTJ8IkV?p=preview (used example from material demo as starting point)

Steps to reproduce

1. show/hide form clicking checkbox.
2. displayed below number of listeners is increasing.
3. there are more and more messages 'Validate called' appears in console when editing input (In my project I had issue when validators set outdated errors, had to use workaround).

There are 3 inputs to show that they are not synchronized (Is there a significant reason to store all closures, not only last?).

What is the motivation/use case for changing the behavior?

1. Allow to have permanent FormControl/FormGroup without memory leaks.
2. Allow to dynamically create FormControlName with custom control, which knows how to validate itself (providing NG_VALIDATORS), without calling validators of destroyed controls.

Environment

Angular version: 4.4.6


Browser:
- [x] Chrome (desktop) version 62.0.3202.75
- [ ] Chrome (Android) version XX
- [ ] Chrome (iOS) version XX
- [ ] Firefox version XX
- [ ] Safari (desktop) version XX
- [ ] Safari (iOS) version XX
- [ ] IE version XX
- [ ] Edge version XX
 
Others: