Snackbar example in material.angular.io breaks after a few fast clicks

[radoslavpetranov]

Bug, feature request, or proposal:

Try double clicking (without pausing) on the example on https://material.angular.io/components/component/snack-bar - after a couple of those you get an exception in the console and the whole app breaks.

What are the steps to reproduce?

Open up the link above, go to examples, try double clicking the "PIZZA PARTY" button 2 or 3 times or just hit it multiple times fast before the message goes down.

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

beta.1

Obviously not a show stopper as nobody would realistically be doing this but I'm just adding the bug report as I noticed the behavior anyway.

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