Minor cleanup

Author: jpnurmi

lib/src/spin_controller.dart

@@ -72,11 +72,10 @@ class SpinController extends ValueNotifier<double> {
     return int.tryParse(text)?.toDouble();
   }
 
-  void stepUp() => _doStep(step);
-  void stepDown() => _doStep(-step);
-  void pageStepUp() => _doStep(pageStep ?? 0);
-  void pageStepDown() => _doStep(-(pageStep ?? 0));
-  void _doStep(double step) => value += step;
+  void stepUp() => value += step;
+  void stepDown() => value += -step;
+  void pageStepUp() => value += pageStep ?? 0;
+  void pageStepDown() => value -= pageStep ?? 0;
 
   bool canChange(double value) => true;