isUniform property Null safety
Whether all four sides of the border are identical. Uniform borders are typically more efficient to paint.
A uniform border by definition has no text direction dependency and therefore could be expressed as a Border, even if it is currently a BorderDirectional. A uniform border can also be expressed as a RoundedRectangleBorder.
@override bool get isUniform => _colorIsUniform && _widthIsUniform && _styleIsUniform;