cdk-global-scrollblock not removed when navigating while dialog is being closed

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

#### What is the expected behavior?
cdk-global-scrollblock is removed from document when the router navigates to a new route while the dialog is being closed (not yet tested if the same happens while navigating when a dialog is open)

#### What is the current behavior?
When navigating while closing the dialog, the class cdk-global-scrollblock will not be removed from document and scrolling is not possible anymore.

#### What are the steps to reproduce?
Open a dialog, call this.mdDialogRef.close() and immediately navigate to a different route. It can be solved easily by observing this.mdDialogRef.afterClosed but I think that the framework should ensure that cdk-global-scrollblock gets removed when navigating.

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

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular: 4.3.0
Material: 2.0.0-beta.8-dd230a3
CDK: 2.0.0-beta.8-dd230a3
OS: Linux
Typescript: 2.3.4
Browsers: Chrome

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

Reproduzieren Sie das Problem, indem Sie einen Dialog öffnen, mdDialogRef.close() aufrufen und sofort zu einer anderen Route navigieren. Verfolgen Sie den Lebenszyklus beim Schließen des Dialogs und die Bereinigung von cdk-global-scrollblock; abgeschlossen ist dies, wenn die Klasse aus document entfernt wurde und das Scrollen nach der Navigation weiterhin möglich ist.

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