[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...)

Comienza con la reproducción vinculada de StackBlitz y reproduce la fuga cambiando entre las rutas uno y dos dentro de sidenav. Compara los snapshots del heap de Chrome antes y después de cambios adicionales; se considera terminado cuando OneComponent y TwoComponent, junto con los objetos relacionados, se liberan en lugar de retenerse.

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