Setting a single formControl to Untouched does not reset the error css state

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
Don't use mat-form-field-invalid, but use the native angular core css classes ng-invalid
Because mat-form-field-invalid does not get removed when setting a single formControl to untouched

#### What is the current behavior?
mat-form-field-invalid does not get removed when doing markAsUntouched

#### What are the steps to reproduce?
Providing a StackBlitz reproduction is the *best* way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

On every simple form

#### What is the use-case or motivation for changing an existing behavior?
Material uses it's own mat-form-field-invalid css class instead of the angular core css classes, this makes error. This is a wrong design

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
newest versions

#### Is there anything else we should know?

Comienza con la reproducción vinculada de StackBlitz y rastrea cómo markAsUntouched afecta al estado de error del campo de formulario de Material frente a las clases nativas de form-control de Angular. Confirma el comportamiento en un formulario sencillo y, después, verifica que marcar un control como untouched elimina el estilo de inválido correspondiente sin cambiar los demás controles.

Escrito por el modelo de indexación a partir del texto del issue.