Add Sample code for FlatButton #21136 (#27751)

* Update flat_button.dart

* Added smaple code for Flat Button

* Update flat_button.dart

* Revert "Update flat_button.dart"

This reverts commit c8f00dff39e18b267295890bd0c5a86979b58c24.

* Revert "Added smaple code for Flat Button"

This reverts commit adc4234bc7956293a5b56f39701c6967fe3e919a.

* Update flat_button.dart

* Update flat_button.dart

* Update flat_button.dart

* remove white spaces

* remove white spaces

* add two different samples

* Update flat_button.dart