Cannot add ngProjectAs to host element as attribute

[dobrevad]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[x] Bug report  
[ ] Performance issue
[ ] 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
[ ] Other... Please describe:

Current behavior

I'm trying to use ngProjectAs by adding it to the host element but it doesn't work.

@Component({
  selector: 'my-form-error',
  template: `...`
  host: {
    '[attr.ngProjectAs]': 'projectAs',
  },
  providers: [{provide: MatError, useValue: MyFormError}],
})
export class MyFormError extends MatError {
  projectAs = "mat-error";
}

The following does but is not what I need:

    <my-error ngProjectAs="mat-error">
      my error message.
    </my-form-error>

Expected behavior

Expect to be able to add ngProjectAs to the host element since it's an attribute.

Is there anything wrong with doing that?

Minimal reproduction of the problem with instructions

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

Environment

Angular version: v6.0.2


Browser:
- [ ] Chrome (desktop) version XX
- [ ] 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
 
For Tooling issues:
- Node version: XX  
- Platform:  < Mac, Linux>

Others:

[dobrevad]

Apparently that's by design since ngProjectAs is evaluated at compile time but is added dynamically to the host element. Resolving.

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