Dart APIunself_componentButtonForegroundonPrimary onPrimary property Null safety Color? onPrimary final Implementation final Color? onPrimary; unself_component unself_component ButtonForeground onPrimary property ButtonForeground class Constructors ButtonForeground Properties hashCode onPrimary onSurface runtimeType Methods debugFillProperties noSuchMethod resolve toDiagnosticsNode toString toStringShort Operators operator ==