Fix `iconTheme` in `AppBar` doesn't apply custom `Colors.white` in the dark mode for M3 (#130574)

fixes [[Material3] AppBar does not respect `foregroundColor` or `iconTheme` for leading and actions in some cases](https://github.com/flutter/flutter/issues/130485)

### Description

- Fix `Colors.white` not applied in dark mode
- Add regression tests
- make `iconStyle` private for consistency

### Before
![Screenshot 2023-07-14 at 18 40 58](https://github.com/flutter/flutter/assets/48603081/a6caffd6-d9a1-407a-aea7-c30047bfe7c7)

### After
![Screenshot 2023-07-14 at 18 41 04](https://github.com/flutter/flutter/assets/48603081/f864da7a-2ff8-46a4-8927-591e50050502)

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

@@ -4,8 +4,6 @@
 
 import 'package:flutter/painting.dart';
 
-import 'colors.dart';
-
 /// The minimum dimension of any interactive region according to Material
 /// guidelines.
 ///
@@ -53,9 +51,11 @@ const EdgeInsets kMaterialListPadding = EdgeInsets.symmetric(vertical: 8.0);
 /// The default color for [ThemeData.iconTheme] when [ThemeData.brightness] is
 /// [Brightness.dark]. This color is used in [IconButton] to detect whether
 /// [IconTheme.of(context).color] is the same as the default color of [ThemeData.iconTheme].
-const Color kDefaultIconLightColor = Colors.white;
+// ignore: prefer_const_constructors
+final Color kDefaultIconLightColor = Color(0xFFFFFFFF);
 
 /// The default color for [ThemeData.iconTheme] when [ThemeData.brightness] is
 /// [Brightness.light]. This color is used in [IconButton] to detect whether
 /// [IconTheme.of(context).color] is the same as the default color of [ThemeData.iconTheme].
-const Color kDefaultIconDarkColor = Colors.black87;
+// ignore: prefer_const_constructors
+final Color kDefaultIconDarkColor = Color(0xDD000000);

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

@@ -963,9 +963,9 @@ class _IconButtonM3 extends ButtonStyleButton {
 
     bool isIconThemeDefault(Color? color) {
       if (isDark) {
-        return color == kDefaultIconLightColor;
+        return identical(color, kDefaultIconLightColor);
       }
-      return color == kDefaultIconDarkColor;
+      return identical(color, kDefaultIconDarkColor);
     }
     final bool isDefaultColor = isIconThemeDefault(iconTheme.color);
     final bool isDefaultSize = iconTheme.size == const IconThemeData.fallback().size;

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

@@ -543,7 +543,7 @@ class ThemeData with Diagnosticable {
     }
     textTheme = defaultTextTheme.merge(textTheme);
     primaryTextTheme = defaultPrimaryTextTheme.merge(primaryTextTheme);
-    iconTheme ??= isDark ? const IconThemeData(color: kDefaultIconLightColor) : const IconThemeData(color: kDefaultIconDarkColor);
+    iconTheme ??= isDark ? IconThemeData(color: kDefaultIconLightColor) : IconThemeData(color: kDefaultIconDarkColor);
     primaryIconTheme ??= primaryIsDark ? const IconThemeData(color: Colors.white) : const IconThemeData(color: Colors.black);
 
     // COMPONENT THEMES

packages/flutter/test/material/app_bar_theme_test.dart

@@ -1055,6 +1055,34 @@ void main() {
     // one is used. This results in a difference for doubles in debugFillProperties between
     // the web and the rest of Flutter's target platforms.
   }, skip: kIsWeb); // https://github.com/flutter/flutter/issues/87364
+
+  // This is a regression test for https://github.com/flutter/flutter/issues/130485.
+  testWidgets('Material3 - AppBarTheme.iconTheme correctly applies custom white color in dark mode', (WidgetTester tester) async {
+    final ThemeData themeData = ThemeData(
+      useMaterial3: true,
+      brightness: Brightness.dark,
+      appBarTheme: const AppBarTheme(iconTheme: IconThemeData(color: Colors.white)),
+    );
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: themeData,
+        home: Scaffold(
+          appBar: AppBar(
+            leading: IconButton(icon: const Icon(Icons.menu), onPressed: () {}),
+            actions: <Widget>[
+              IconButton(icon: const Icon(Icons.add), onPressed: () {}),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    Color? leadingIconButtonColor() => _iconStyle(tester, Icons.menu)?.color;
+    Color? actionIconButtonColor() => _iconStyle(tester, Icons.add)?.color;
+
+    expect(leadingIconButtonColor(), Colors.white);
+    expect(actionIconButtonColor(), Colors.white);
+  });
 }
 
 AppBarTheme _appBarTheme() {