fix(demo-app) icon inside a button in the toolbar

[ghost]

fixes #418

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

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• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
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[jelbourn]

@mmgdort LGTM, we just need you to sign the CLA.

[ghost]

done!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!