[Sidenav] Memory leak when used inside router-outlet

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

When a sidenav is used "inside" router-outlet, it seems there is a memory leak. When the router-outlet is inside the sidenav, there is no problem.

I use the "Take heap snapshot" of Chrome to see the problem.

#### What are the steps to reproduce?

Sample available on:

https://stackblitz.com/edit/angular-material2-issue-xmgukp?file=app%2Fapp.component.ts

Open in Chrome:

https://angular-material2-issue-xmgukp.stackblitz.io/

- Switch between "one" and "two" routes a few times.
- Take a heap snapshot
- Switch again between routes
- Take another snapshot and compare => OneComponent and TwoComponent are not freed (among many other objects...)

Start with the linked StackBlitz reproduction and reproduce the leak by switching between the one and two routes inside the sidenav. Compare Chrome heap snapshots before and after additional switches; done means OneComponent and TwoComponent, along with related objects, are released instead of retained.

Written by the indexing model from the issue text.