Removed primaryVariant from usage by the SnackBar. (#92443)

packages/flutter/lib/src/material/snack_bar.dart

@@ -421,7 +421,7 @@ class _SnackBarState extends State<SnackBar> {
     final ColorScheme colorScheme = theme.colorScheme;
     final SnackBarThemeData snackBarTheme = theme.snackBarTheme;
     final bool isThemeDark = theme.brightness == Brightness.dark;
-    final Color buttonColor = isThemeDark ? colorScheme.primaryVariant : colorScheme.secondary;
+    final Color buttonColor = isThemeDark ? colorScheme.primary : colorScheme.secondary;
 
     // SnackBar uses a theme that is the opposite brightness from
     // the surrounding theme.
@@ -433,8 +433,6 @@ class _SnackBarState extends State<SnackBar> {
       colorScheme: ColorScheme(
         primary: colorScheme.onPrimary,
         primaryVariant: colorScheme.onPrimary,
-        // For the button color, the spec says it should be primaryVariant, but for
-        // backward compatibility on light themes we are leaving it as secondary.
         secondary: buttonColor,
         secondaryVariant: colorScheme.onSecondary,
         surface: colorScheme.onSurface,

packages/flutter/test/material/snack_bar_test.dart

@@ -667,6 +667,45 @@ void main() {
     expect(renderModel.color, equals(darkTheme.colorScheme.onSurface));
   });
 
+  testWidgets('Dark theme SnackBar has primary text buttons', (WidgetTester tester) async {
+    final ThemeData darkTheme = ThemeData.dark();
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: darkTheme,
+        home: Scaffold(
+          body: Builder(
+            builder: (BuildContext context) {
+              return GestureDetector(
+                onTap: () {
+                  Scaffold.of(context).showSnackBar(
+                    SnackBar(
+                      content: const Text('I am a snack bar.'),
+                      duration: const Duration(seconds: 2),
+                      action: SnackBarAction(
+                        label: 'ACTION',
+                        onPressed: () { },
+                      ),
+                    ),
+                  );
+                },
+                child: const Text('X'),
+              );
+            },
+          ),
+        ),
+      ),
+    );
+
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+
+    final TextStyle buttonTextStyle = tester.widget<RichText>(
+        find.descendant(of: find.text('ACTION'), matching: find.byType(RichText))
+    ).text.style!;
+    expect(buttonTextStyle.color, equals(darkTheme.colorScheme.primary));
+  });
+
   testWidgets('SnackBar should inherit theme data from its ancestor.', (WidgetTester tester) async {
     final SliderThemeData sliderTheme = SliderThemeData.fromPrimaryColors(
       primaryColor: Colors.black,