make CupertinoDynamicColor const constructible (#39430)

* make CupertinoDynamicColor const constructible

* review

packages/flutter/test/cupertino/slider_test.dart

@@ -11,6 +11,17 @@ import 'package:flutter_test/flutter_test.dart';
 import '../rendering/mock_canvas.dart';
 import '../widgets/semantics_tester.dart';
 
+const CupertinoDynamicColor _kSystemFill = CupertinoDynamicColor(
+  color: Color.fromARGB(51, 120, 120, 128),
+  darkColor: Color.fromARGB(91, 120, 120, 128),
+  highContrastColor: Color.fromARGB(71, 120, 120, 128),
+  darkHighContrastColor: Color.fromARGB(112, 120, 120, 128),
+  elevatedColor: Color.fromARGB(51, 120, 120, 128),
+  darkElevatedColor: Color.fromARGB(91, 120, 120, 128),
+  highContrastElevatedColor: Color.fromARGB(71, 120, 120, 128),
+  darkHighContrastElevatedColor: Color.fromARGB(112, 120, 120, 128),
+);
+
 void main() {
 
   Future<void> _dragSlider(WidgetTester tester, Key sliderKey) {
@@ -441,15 +452,15 @@ void main() {
   });
 
   testWidgets('Themes can be overridden by dynamic colors', (WidgetTester tester) async {
-    final CupertinoDynamicColor activeColor = CupertinoDynamicColor(
-      color: const Color(0x00000001),
-      darkColor: const Color(0x00000002),
-      elevatedColor: const Color(0x00000003),
-      highContrastColor: const Color(0x00000004),
-      darkElevatedColor: const Color(0x00000005),
-      darkHighContrastColor: const Color(0x00000006),
-      highContrastElevatedColor: const Color(0x00000007),
-      darkHighContrastElevatedColor: const Color(0x00000008),
+    const CupertinoDynamicColor activeColor = CupertinoDynamicColor(
+      color: Color(0x00000001),
+      darkColor: Color(0x00000002),
+      elevatedColor: Color(0x00000003),
+      highContrastColor: Color(0x00000004),
+      darkElevatedColor: Color(0x00000005),
+      darkHighContrastColor: Color(0x00000006),
+      highContrastElevatedColor: Color(0x00000007),
+      darkHighContrastElevatedColor: Color(0x00000008),
     );
 
     Widget withTraits(Brightness brightness, CupertinoUserInterfaceLevelData level, bool highContrast) {
@@ -512,12 +523,12 @@ void main() {
 
     expect(
       find.byType(CupertinoSlider),
-      paints..rrect(color: CupertinoSystemColors.fallbackValues.systemFill.color),
+      paints..rrect(color: _kSystemFill.color),
     );
 
     expect(
       find.byType(CupertinoSlider),
-      isNot(paints..rrect(color: CupertinoSystemColors.fallbackValues.systemFill.darkColor)),
+      isNot(paints..rrect(color: _kSystemFill.darkColor)),
     );
 
     await tester.pumpWidget(
@@ -535,12 +546,12 @@ void main() {
 
     expect(
       find.byType(CupertinoSlider),
-      paints..rrect(color: CupertinoSystemColors.fallbackValues.systemFill.darkColor),
+      paints..rrect(color: _kSystemFill.darkColor),
     );
 
     expect(
       find.byType(CupertinoSlider),
-      isNot(paints..rrect(color: CupertinoSystemColors.fallbackValues.systemFill.color)),
+      isNot(paints..rrect(color: _kSystemFill.color)),
     );
   });
 }