emerald top-level property Null safety

Implementation

NBColor emerald = const NBColor(
    0xff10b981,
    <String, Color>{
      'emerald.50 ': Color(0xffecfdf5),
      'emerald.100': Color(0xffd1fae5),
      'emerald.200': Color(0xffa7f3d0),
      'emerald.300': Color(0xff6ee7b7),
      'emerald.400': Color(0xff34d399),
      'emerald.500': Color(0xff10b981),
      'emerald.600': Color(0xff059669),
      'emerald.700': Color(0xff047857),
      'emerald.800': Color(0xff065f46),
      'emerald.900': Color(0xff064e3b),
    },
    'emerald');