Remove extra padding if a dropdown menu entry also has a leading icon (#135004)

Fixes #131350

This PR is to remove the extra padding in `DropdownMenuEntry` if both the text field and the dropdown menu entry have leading icons.

**After** fix:
<img width="300" alt="Screenshot 2023-09-19 at 4 35 24 PM" src="https://github.com/flutter/flutter/assets/36861262/ed7d92a5-3f96-4106-a03e-09258ea3709f">

**Before** fix:
<img width="300" alt="Screenshot 2023-09-19 at 4 37 58 PM" src="https://github.com/flutter/flutter/assets/36861262/fdbfef54-6c93-48fb-bd64-41fa31dde531">

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

@@ -432,21 +432,28 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
     { int? focusedIndex, bool enableScrollToHighlight = true}
   ) {
     final List<Widget> result = <Widget>[];
-    final double padding = leadingPadding ?? _kDefaultHorizontalPadding;
-    final ButtonStyle defaultStyle;
-    switch (textDirection) {
-      case TextDirection.rtl:
-        defaultStyle = MenuItemButton.styleFrom(
-          padding: EdgeInsets.only(left: _kDefaultHorizontalPadding, right: padding),
-        );
-      case TextDirection.ltr:
-        defaultStyle = MenuItemButton.styleFrom(
-          padding: EdgeInsets.only(left: padding, right: _kDefaultHorizontalPadding),
-        );
-    }
-
     for (int i = 0; i < filteredEntries.length; i++) {
       final DropdownMenuEntry<T> entry = filteredEntries[i];
+
+      // By default, when the text field has a leading icon but a menu entry doesn't
+      // have one, the label of the entry should have extra padding to be aligned
+      // with the text in the text input field. When both the text field and the
+      // menu entry have leading icons, the menu entry should remove the extra
+      // paddings so its leading icon will be aligned with the leading icon of
+      // the text field.
+      final double padding = entry.leadingIcon == null ? (leadingPadding ?? _kDefaultHorizontalPadding) : _kDefaultHorizontalPadding;
+      final ButtonStyle defaultStyle;
+      switch (textDirection) {
+        case TextDirection.rtl:
+          defaultStyle = MenuItemButton.styleFrom(
+            padding: EdgeInsets.only(left: _kDefaultHorizontalPadding, right: padding),
+          );
+        case TextDirection.ltr:
+          defaultStyle = MenuItemButton.styleFrom(
+            padding: EdgeInsets.only(left: padding, right: _kDefaultHorizontalPadding),
+          );
+      }
+
       ButtonStyle effectiveStyle = entry.style ?? defaultStyle;
       final Color focusedBackgroundColor = effectiveStyle.foregroundColor?.resolve(<MaterialState>{MaterialState.focused})
         ?? Theme.of(context).colorScheme.onSurface;

packages/flutter/test/material/dropdown_menu_test.dart

@@ -1680,6 +1680,40 @@ void main() {
     await tester.pumpAndSettle();
     expect(controller.text, ''); // nothing selected
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/131350.
+  testWidgets('DropdownMenuEntry.leadingIcon default layout', (WidgetTester tester) async {
+    // The DropdownMenu should not get extra padding in DropdownMenuEntry items
+    // when both text field and DropdownMenuEntry have leading icons.
+    await tester.pumpWidget(const MaterialApp(
+        home: Scaffold(
+          body: DropdownMenu<int>(
+            leadingIcon: Icon(Icons.search),
+            hintText: 'Hint',
+            dropdownMenuEntries: <DropdownMenuEntry<int>>[
+              DropdownMenuEntry<int>(
+                value: 0,
+                label: 'Item 0',
+                leadingIcon: Icon(Icons.alarm)
+              ),
+              DropdownMenuEntry<int>(value: 1, label: 'Item 1'),
+            ],
+          ),
+        )
+    ));
+    await tester.tap(find.byType(DropdownMenu<int>));
+    await tester.pumpAndSettle();
+
+    // Check text location in text field.
+    expect(tester.getTopLeft(find.text('Hint')).dx, 48.0);
+
+    // By default, the text of item 0 should be aligned with the text of the text field.
+    expect(tester.getTopLeft(find.text('Item 0').last).dx, 48.0);
+
+    // By default, the text of item 1 should be aligned with the text of the text field,
+    // so there are some extra padding before "Item 1".
+    expect(tester.getTopLeft(find.text('Item 1').last).dx, 48.0);
+  });
 }
 
 enum TestMenu {