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?

ダイアログを開き、mdDialogRef.close() を呼び出して、直ちに別の route へ移動することで問題を再現します。ダイアログの終了ライフサイクルと cdk-global-scrollblock のクリーンアップを追跡します。完了の条件は、クラスが document から削除され、移動後もスクロールできることです。

索引モデルが issue の本文から書いたものです。