A required mat-input field does not show error when the parent form is submitted.

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

For the purpose of code re-use and to maintain consistent functionality across my web app, I created an input component in my app whose role is to embed a mat-input in a parent form.

When the mat-input is a required field, the component displays it correctly in the parent form and an error is displayed when a user interacts with the field but does not enter anything. However, no error is displayed when the parent form is submitted without any user interaction with the embedded field.

Am I missing something? Is there a workaround?

#### What is the expected behavior?

An error should be displayed when the parent form is submitted.

#### What is the current behavior?

An error is not displayed when the parent form is submitted.

#### What are the steps to reproduce?

See: https://stackblitz.com/edit/angular-material2-issue-jph57r

#### What is the use-case or motivation for changing an existing behavior?

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

I am using Chrome. Output of ng --version is given below:

```
Angular CLI: 1.6.0
Node: 8.9.3
OS: win32 x64
Angular: 5.1.0
... animations, common, compiler, compiler-cli, core, forms
... http, platform-browser, platform-browser-dynamic
... platform-server, router

@angular/cdk: 5.0.0
@angular/cli: 1.6.0
@angular/language-service: 4.4.6
@angular/material: 5.0.0
@angular-devkit/build-optimizer: 0.0.38
@angular-devkit/core: 0.0.25
@angular-devkit/schematics: 0.0.48
@ngtools/json-schema: 1.1.0
@ngtools/webpack: 1.9.0
@schematics/angular: 0.1.13
@schematics/schematics: 0.0.13
typescript: 2.4.2
webpack: 3.10.0

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

リンクされている StackBlitz の再現手順から始め、埋め込まれた mat-input が親フォームにどのように接続され、操作なしで送信されるかを確認します。発生していないバリデーションエラーを再現し、その後、親フォームを送信したときに必須フィールドのエラーが表示されることを確認します。

索引モデルが issue の本文から書いたものです。