Fix IconButton.color overrided by IconButtomTheme (flutter#174515)

## Description

This PR fixes IconButton icon color resolution. 

## Before

The IconButton.color property does not take precedence over the ambient
`IconButtonTheme`

## After

The IconButton.color property overrides the ambient `IconButtonTheme`.

## Related Issue

Fixes [SnackBar closeIconColor does not respect widget
override](flutter#174472)
Fixes [IconButton.color is overidden by
IconButtonTheme](flutter#174511)

## Tests

Adds 1 test.

Author: bleroux

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

@@ -745,7 +745,9 @@ class IconButton extends StatelessWidget {
       if (style != null) {
         adjustedStyle = style!.merge(adjustedStyle);
       }
-
+      if (adjustedStyle.iconColor == null) {
+        adjustedStyle = adjustedStyle.copyWith(iconColor: adjustedStyle.foregroundColor);
+      }
       Widget effectiveIcon = icon;
       if ((isSelected ?? false) && selectedIcon != null) {
         effectiveIcon = selectedIcon!;

packages/flutter/test/material/icon_button_test.dart

@@ -2761,6 +2761,37 @@ void main() {
     expect(getOverlayColor(tester), paints..rect(color: overlayColor));
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/174511.
+  testWidgets('IconButton.color takes precedence over ambient IconButtonThemeData.iconColor', (
+    WidgetTester tester,
+  ) async {
+    const Color iconButtonColor = Color(0xFFFF1234);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(
+          iconButtonTheme: const IconButtonThemeData(
+            style: ButtonStyle(
+              iconColor: WidgetStateColor.fromMap(<WidgetStatesConstraint, Color>{
+                WidgetState.any: Colors.purple,
+              }),
+            ),
+          ),
+        ),
+        home: Material(
+          child: Center(
+            child: IconButton(
+              onPressed: () {},
+              icon: const Icon(Icons.add, size: 64),
+              color: iconButtonColor,
+            ),
+          ),
+        ),
+      ),
+    );
+    expect(_iconStyle(tester, Icons.add)?.color, iconButtonColor);
+  });
+
   group('IconTheme tests in Material 3', () {
     testWidgets('IconTheme overrides default values in M3', (WidgetTester tester) async {
       // Theme's IconTheme