inconsistent mat-snackbar screenreader behavior

[diminutivesloop]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Screen readers should announce snack-bar messages.

What is the current behavior?

When using JAWS or NVDA with Chrome, if a mat-snackbar is invoked with a custom component and the announcementMessage option set, both the text content of the custom snack-bar and the announcementMessage string are announced to the user. However when using NVDA with other browsers such as IE, or Firefox only the announcementMessage string is announced. One way to work around this issue is to hide the custom snackbar from the screenreader via aria-hidden but this prevents the snackbar from being read to the user if they manually navigate to it.

What are the steps to reproduce?

https://stackblitz.com/edit/angular-gbkaav-pka2ed

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

Angular: 6.1.7
Material: 6.4.7
NVDA: 2017.4
JAWS: 18.0.4104
Chrome: 68.0.3440.106
IE: 11.0.9600.18499
Firefox: 62.0

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