center property
The color that correlates to stops entry 0 (the first color) is placed
at AlignmentGeometry center, an offset into the (-1,-1) x (1,1)
"paintbox" onto which the gradient will be painted.
For example, center: Alignment(0,0) will place the radial gradient in
the center of this paintbox. This is a standard known as
Alignment.center.
Implementation
final AlignmentGeometry center;