Snackbar blocks dialog close via escape

[diminutivesloop]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Pressing the escape key while a dialog is open should close the dialog.

What is the current behavior?

If a snackbar is open at the same time as a dialog pressing the escape key does not close the dialog until the snackbar is dismissed.

What are the steps to reproduce?

https://stackblitz.com/edit/dialog-snackbar-esc-close

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

@angular/core 6.0.5
@angular/material 6.2.1
Windows 7
Chrome 67.0.3396.87
Firefox 60.0.1
IE 11.0.9600.18499

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