EdgeInsets.symmetric constructor

const EdgeInsets.symmetric({double vertical: 0.0, double horizontal: 0.0 })

Creates insets with symmetrical vertical and horizontal offsets.

Eight pixel margin above and below, no horizontal margins: 
const EdgeInsets.symmetric(vertical: 8.0)

Implementation 

const EdgeInsets.symmetric({ double vertical = 0.0,
                           double horizontal = 0.0 })
  : left = horizontal, top = vertical, right = horizontal, bottom = vertical;