form-field: support swapping various child components with ngIf

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

I recently ran into a bug that `mat-hint` threw an error when its corresponding `matInput` is inserted conditionally. E.g. take the following template:

```

Some hint.

```
Note that the only reason inputs are inserted conditionally here is because the placeholder text is different depending on the condition.

This will throw the following error:

```
app.component.html:7 ERROR Error: ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked. Previous value: 'null'. Current value: 'mat-hint-0'.
at new d (VM8254 zone.min.js:1)
at viewDebugError (VM7606 core.umd.js:8466)
at expressionChangedAfterItHasBeenCheckedError (VM7606 core.umd.js:8444)
at checkBindingNoChanges (VM7606 core.umd.js:8608)
at checkNoChangesNodeDynamic (VM7606 core.umd.js:12515)
at checkNoChangesNode (VM7606 core.umd.js:12464)
at debugCheckNoChangesNode (VM7606 core.umd.js:13241)
at debugCheckRenderNodeFn (VM7606 core.umd.js:13181)
at Object.eval [as updateRenderer] (VM7993 AppComponent.ngfactory.js:78)
at Object.debugUpdateRenderer [as updateRenderer] (VM7606 core.umd.js:13163)
at checkNoChangesView (VM7606 core.umd.js:12283)
at callViewAction (VM7606 core.umd.js:12650)
at execEmbeddedViewsAction (VM7606 core.umd.js:12628)
at checkNoChangesView (VM7606 core.umd.js:12282)
at callViewAction (VM7606 core.umd.js:12650)
```
This is not a problem when the input is not inserted conditionally but rather placed in the template right away. **I'm not sure if that is by design or not** but it caused me some hours of debugging until I realised it was the combination of `mat-hint` inside a `mat-form-field` whom\s `matInput` is rendered using `*ngIf`.

#### What is the expected behavior?

I expected this to just work, but again, maybe this is by design but then we should document somewhere that `matInput`s aren't allowed to be inserted using structural directives.

#### What is the current behavior?

Described above.

#### What are the steps to reproduce?

Here's a plunk that reproduces the error: http://plnkr.co/edit/MrUvFVKIW1BoX9d96eeu?p=preview

Simply check the console.

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

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

I'm using `@angular/material` version `2.0.0-beta.11` but the plunk uses the latest version. Unfortunately I can't tell if this error has been thrown in older versions as well.

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

Yeah, you're all doing an amazing job. Thanks for that.

Beginne mit dem Beispiel in der Vorlage des Issues und dem verlinkten Plunker und reproduziere den ExpressionChangedAfterItHasBeenCheckedError, wenn matInput-Kinder zusammen mit mat-hint mit *ngIf eingefügt werden. Lies das Verhalten von mat-form-field, matInput und mat-hint, das an den bedingten Änderungen der Kind-Elemente beteiligt ist. Als abgeschlossen gilt die Aufgabe, wenn das bedingte Austauschen der Inputs den gemeldeten Fehler nicht mehr auslöst und der reproduzierte Fall durch Tests abgedeckt ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.