Fix TabBar tab icons not respecting custom IconTheme (#157724)

Fixes [TabBar ignores Theme's iconTheme.size](https://github.com/flutter/flutter/issues/155518)

### Description

When `ThemeData.IconTheme`  with color and size, is provided, it can override the default `Tab` icon color and size. 

### Code sample

<details>
<summary>expand to view the code sample</summary> 

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        iconTheme: const IconThemeData(color: Colors.amber, size: 40),
      ),
      home: const Scaffold(
        body: SafeArea(
          child: DefaultTabController(
            length: 2,
            child: Column(
              children: [
                TabBar(
                  tabs: [
                    Tab(icon: Icon(Icons.backpack_outlined), text: 'Backpack'),
                    Tab(icon: Icon(Icons.map_outlined), text: 'Map'),
                  ],
                  overlayColor: WidgetStatePropertyAll(Colors.transparent),
                ),
                Expanded(
                  child: TabBarView(
                    children: [
                      Icon(Icons.backpack_outlined),
                      Icon(Icons.map_outlined),
                    ],
                  ),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}
```

</details>

---

<img width="666" alt="Screenshot 2024-10-28 at 16 25 06" src="https://github.com/user-attachments/assets/56d31115-7ee9-4378-9811-75a3a2a4ce0f">

<img width="666" alt="Screenshot 2024-10-28 at 16 24 52" src="https://github.com/user-attachments/assets/ab8a3e4b-e912-40ac-b249-6358492581e0">

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

@@ -269,7 +269,10 @@ class _TabStyle extends AnimatedWidget {
   final TabBarThemeData defaults;
   final Widget child;
 
-  MaterialStateColor _resolveWithLabelColor(BuildContext context) {
+  MaterialStateColor _resolveWithLabelColor(
+    BuildContext context, {
+    IconThemeData? iconTheme,
+  }) {
     final ThemeData themeData = Theme.of(context);
     final TabBarThemeData tabBarTheme = TabBarTheme.of(context);
     final Animation<double> animation = listenable as Animation<double>;
@@ -295,6 +298,7 @@ class _TabStyle extends AnimatedWidget {
           ?? tabBarTheme.unselectedLabelColor
           ?? unselectedLabelStyle?.color
           ?? tabBarTheme.unselectedLabelStyle?.color
+          ?? iconTheme?.color
           ?? (themeData.useMaterial3
                ? defaults.unselectedLabelColor!
                : selectedColor.withAlpha(0xB2)); // 70% alpha
@@ -310,6 +314,7 @@ class _TabStyle extends AnimatedWidget {
 
   @override
   Widget build(BuildContext context) {
+    final ThemeData theme = Theme.of(context);
     final TabBarThemeData tabBarTheme = TabBarTheme.of(context);
     final Animation<double> animation = listenable as Animation<double>;
 
@@ -331,14 +336,23 @@ class _TabStyle extends AnimatedWidget {
     final TextStyle textStyle = isSelected
       ? TextStyle.lerp(selectedStyle, unselectedStyle, animation.value)!
       : TextStyle.lerp(unselectedStyle, selectedStyle, animation.value)!;
-    final Color color = _resolveWithLabelColor(context).resolve(states);
+    final Color defaultIconColor = switch (theme.colorScheme.brightness) {
+      Brightness.light => kDefaultIconDarkColor,
+      Brightness.dark  => kDefaultIconLightColor,
+    };
+    final IconThemeData? customIconTheme = switch (IconTheme.of(context)) {
+      final IconThemeData iconTheme when iconTheme.color != defaultIconColor => iconTheme,
+      _ => null,
+    };
+    final Color iconColor = _resolveWithLabelColor(context, iconTheme: customIconTheme).resolve(states);
+    final Color labelColor = _resolveWithLabelColor(context).resolve(states);
 
     return DefaultTextStyle(
-      style: textStyle.copyWith(color: color),
+      style: textStyle.copyWith(color: labelColor),
       child: IconTheme.merge(
         data: IconThemeData(
-          size: 24.0,
-          color: color,
+          size: customIconTheme?.size ?? 24.0,
+          color: iconColor,
         ),
         child: child,
       ),

packages/flutter/test/material/tabs_test.dart

@@ -7464,4 +7464,56 @@ void main() {
     targetRect = const Rect.fromLTRB(275.0, 0.0, 325.0, 48.0);
     expectIndicatorAttrs(tabBarBox, rect: rect, targetRect: targetRect);
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/155518.
+  testWidgets('Tabs icon respects ambient icon theme', (WidgetTester tester) async {
+    final ThemeData theme = ThemeData(
+      iconTheme: const IconThemeData(
+        color: Color(0xffff0000),
+        size: 38.0,
+      ),
+    );
+    const IconData selectedIcon = Icons.ac_unit;
+    const IconData unselectedIcon = Icons.access_alarm;
+    await tester.pumpWidget(boilerplate(
+      theme: theme,
+      child: const DefaultTabController(
+        length: 2,
+        child: TabBar(
+          tabs: <Widget>[
+            Tab(
+              icon: Icon(selectedIcon),
+              text: 'Tab 1',
+            ),
+            Tab(
+              icon: Icon(unselectedIcon),
+              text: 'Tab 2',
+            ),
+          ],
+        ),
+      ),
+    ));
+
+    TextStyle iconStyle(WidgetTester tester, IconData icon) {
+      final RichText iconRichText = tester.widget<RichText>(
+        find.descendant(of: find.byIcon(icon), matching: find.byType(RichText)),
+      );
+      return iconRichText.text.style!;
+    }
+
+    // The iconTheme color isn't applied to the selected icon.
+    expect(iconStyle(tester, selectedIcon).color, equals(theme.colorScheme.primary));
+    // The iconTheme color is applied to the unselected icon.
+    expect(iconStyle(tester, unselectedIcon).color, equals(theme.iconTheme.color));
+
+    // Both selected and unselected icons should have the iconTheme size.
+    expect(
+      tester.getSize(find.byIcon(selectedIcon)),
+      Size(theme.iconTheme.size!, theme.iconTheme.size!),
+    );
+    expect(
+      tester.getSize(find.byIcon(unselectedIcon)),
+      Size(theme.iconTheme.size!, theme.iconTheme.size!),
+    );
+  });
 }