Feature request: automatically use correct case for dialog actions

Action labels on iOS use sentence case whereas on Material Design they are all upper case. It would be really useful to be able to supply Flutter Dialogs with a sentence case String which can be used directly on iOS, but automatically set to upper case when on Android.

A good example can be found on the [Material Design Guidelines](https://material.io/components/dialogs#usage) site if you scroll down to Cross Platform Adaptations.

Thank you.