iOS support

iOS continues to be a big investment area for Flutter.
With this release, we’ve made a visual refresh to our
Cupertino library to match the iOS 13 look. We now support
dark mode in the Cupertino widgets, added two new widgets called
CupertinoContextMenu
and CupertinoSlidingSegmentedControl,
and made improvements to segmented control widgets,
CupertinoAlertDialog,
and CupertinoDatePicker

36871 Audit use of defaultTargetPlatform

37719 CupertinoDynamicColor and friends

38712 Show process error when iOS install fails

39056 Fixed issues with Background Color #34741

39215 CupertinoActionSheet dark mode & fidelity

39289 CupertinoActivityIndicator & CupertinoApp dark mode