invert top-level constant

Implementation

const ColorFilter invert = ColorFilter.matrix(<double>[
  -1,
  0,
  0,
  0,
  255,
  0,
  -1,
  0,
  0,
  255,
  0,
  0,
  -1,
  0,
  255,
  0,
  0,
  0,
  1,
  0,
]);