kCupertinoButtonSizeBorderRadius top-level property
final
        The border radius values for the different CupertinoButtonSizes.
Based on the iOS (17) Human Interface Guidelines.
Implementation
final Map<CupertinoButtonSize, BorderRadius> kCupertinoButtonSizeBorderRadius =
    <CupertinoButtonSize, BorderRadius>{
      CupertinoButtonSize.small: BorderRadius.circular(40),
      CupertinoButtonSize.medium: BorderRadius.circular(40),
      CupertinoButtonSize.large: BorderRadius.circular(12),
    };