MarginSpec.fixedPixel constructor
      
      MarginSpec.fixedPixel(
    
- int? pixels
Create MarginSpec with a fixed pixel size pixels.
pixels if set must be greater than or equal to 0.
Implementation
factory MarginSpec.fixedPixel(int? pixels) {
  // Require require or higher setting if set
  assert(pixels == null || pixels >= 0);
  return MarginSpec._internal(pixels, pixels, null, null);
}