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.

Start with the issue's template example and the linked Plunker, reproducing the ExpressionChangedAfterItHasBeenCheckedError when matInput children are inserted with *ngIf alongside mat-hint. Read the mat-form-field, matInput, and mat-hint behavior involved in conditional child changes. Done means swapping the inputs conditionally no longer throws the reported error, with coverage for the reproduced case.

Written by the indexing model from the issue text.