Complete Paint method.

ArcPainter4.dart

class ArcPainter extends CustomPainter {
  // ...
  
  @override
  void paint(Canvas canvas, Size size) {
    if (backgroundColor != null) {
      final Paint backgroundPaint = Paint()
        ..color = backgroundColor
        ..strokeWidth = strokeWidth
        ..style = PaintingStyle.stroke;
      canvas.drawArc(
          Offset(-strokeWidth / 2, -strokeWidth / 2) &
              Size(size.width + strokeWidth, size.height + strokeWidth),
          0,
          _completeCircumference,
          false,
          backgroundPaint);
    }

    final Paint paint = Paint()
      ..color = color
      ..strokeWidth = strokeWidth
      ..style = PaintingStyle.stroke
      ..strokeCap = StrokeCap.square;

    canvas.drawArc(
        Offset(-strokeWidth / 2, -strokeWidth / 2) &
            Size(size.width + strokeWidth, size.height + strokeWidth),
        arcStart,
        arcSweep,
        false,
        paint);
  }
}