MaterialBasedCupertinoThemeData isn't defined for the class PlatformApp

Hi
I try use `PlatformApp` but i got this error message.

> Error: The method 'MaterialBasedCupertinoThemeData' isn't defined for the class '#lib1::PlatformApp'.
> Try correcting the name to the name of an existing method, or defining a method named 'MaterialBasedCupertinoThemeData'.
>             MaterialBasedCupertinoThemeData(materialTheme: unifiedTheme),
> 