How to close Dialog using FlutterDriver

The key that is commonly used for modals in Flutter is ModalBarrier, which is why the following should do the trick:

await driver.tap(find.byType('ModalBarrier'));

This will work as long as barrierDismissible is set to true.
Essentially, when tapping away a dialog in Flutter, you are tapping on the modal barrier, which is why above code works.

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Thanks to John Muchow for finding out.