Fix Material3 TabBarTheme.dividerColor not working (#119690)

* Fixed Material3 TabBarTheme.dividerColor not working

* Add 'Material3 - TabBar inherits the dividerColor of TabBarTheme' test

---------
Co-authored-by: Kate Lovett <katelovett@google.com>

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

@@ -1122,7 +1122,7 @@ class _TabBarState extends State<TabBar> {
       indicatorPadding: widget.indicatorPadding,
       tabKeys: _tabKeys,
       old: _indicatorPainter,
-      dividerColor: theme.useMaterial3 ? widget.dividerColor ?? defaults.dividerColor : null,
+      dividerColor: theme.useMaterial3 ? widget.dividerColor ?? tabBarTheme.dividerColor ?? defaults.dividerColor : null,
       labelPaddings: _labelPaddings,
     );
   }

packages/flutter/test/material/tabs_test.dart

@@ -5614,6 +5614,36 @@ void main() {
         labelColor.withAlpha(0xB2) // 70% alpha,
       );
     });
+
+    testWidgets('Material3 - TabBar inherits the dividerColor of TabBarTheme', (WidgetTester tester) async {
+      const Color dividerColor = Colors.yellow;
+
+      await tester.pumpWidget(
+        MaterialApp(
+          theme: ThemeData(
+            useMaterial3: true,
+            tabBarTheme: const TabBarTheme(dividerColor: dividerColor),
+          ),
+          home: Scaffold(
+            appBar: AppBar(
+              bottom: TabBar(
+                controller: TabController(length: 3, vsync: const TestVSync()),
+                tabs: const <Widget>[
+                  Tab(text: 'Tab 1'),
+                  Tab(text: 'Tab 2'),
+                  Tab(text: 'Tab 3'),
+                ],
+              ),
+            ),
+          ),
+        ),
+      );
+
+      // Test painter's divider color.
+      final CustomPaint paint = tester.widget<CustomPaint>(find.byType(CustomPaint).last);
+      // ignore: avoid_dynamic_calls
+      expect((paint.painter as dynamic).dividerColor, dividerColor);
+    });
   });
 }