[CupertinoListSection] adds new property separatorColor (#124803)

Adds new property `separatorColor` to `CupertinoListSection` widget.

Fixes: #124744

### `separatorColor` property is null

<img src="https://user-images.githubusercontent.com/13456345/231840875-22605c23-ee9f-464f-a005-c9fce1b641a7.png" width=350>

### `separatorColor` property is not-null

| Color.fromARGB(255, 136, 13, 207) | Color.fromARGB(255, 13, 62, 207) | Color.fromARGB(255, 29, 207, 13) |
| --- | --- | --- |
| ![a](https://user-images.githubusercontent.com/13456345/231840832-6027cf86-e9c0-4206-b6f8-23556471c1bd.png) | ![b](https://user-images.githubusercontent.com/13456345/231840862-dcc30212-14f8-44af-a8cb-d04b22b7a078.png) | ![c](https://user-images.githubusercontent.com/13456345/231840870-d9fafe23-e9a4-423a-a25c-b71835991d07.png) |

packages/flutter/lib/src/cupertino/list_section.dart

@@ -211,6 +211,7 @@ class CupertinoListSection extends StatelessWidget {
     double? additionalDividerMargin,
     this.topMargin = _kMarginTop,
     bool hasLeading = true,
+    this.separatorColor,
   }) : assert((children != null && children.length > 0) || header != null),
        type = CupertinoListSectionType.base,
        additionalDividerMargin = additionalDividerMargin ??
@@ -274,6 +275,7 @@ class CupertinoListSection extends StatelessWidget {
     double? additionalDividerMargin,
     this.topMargin,
     bool hasLeading = true,
+    this.separatorColor,
   }) : assert((children != null && children.length > 0) || header != null),
        type = CupertinoListSectionType.insetGrouped,
        additionalDividerMargin = additionalDividerMargin ??
@@ -344,9 +346,15 @@ class CupertinoListSection extends StatelessWidget {
   /// matches iOS style by default.
   final double? topMargin;
 
+  /// Sets the color for the dividers between rows, and borders on top and
+  /// bottom of the rows.
+  ///
+  /// If null, defaults to [CupertinoColors.separator].
+  final Color? separatorColor;
+
   @override
   Widget build(BuildContext context) {
-    final Color dividerColor = CupertinoColors.separator.resolveFrom(context);
+    final Color dividerColor = separatorColor ?? CupertinoColors.separator.resolveFrom(context);
     final double dividerHeight = 1.0 / MediaQuery.devicePixelRatioOf(context);
 
     // Long divider is used for wrapping the top and bottom of rows.

packages/flutter/test/cupertino/list_section_test.dart

@@ -176,4 +176,50 @@ void main() {
 
     expect(find.byType(ClipRRect), findsNothing);
   });
+
+  testWidgets('CupertinoListSection respects separatorColor', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Center(
+          child: CupertinoListSection(
+            separatorColor: const Color.fromARGB(255, 143, 193, 51),
+            children: const <Widget>[
+              CupertinoListTile(title: Text('CupertinoListTile')),
+              CupertinoListTile(title: Text('CupertinoListTile')),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    final Column childrenColumn = tester.widget(find.byType(Column).at(1));
+    for (final Widget e in childrenColumn.children) {
+      if (e is Container) {
+        expect(e.color, const Color.fromARGB(255, 143, 193, 51));
+      }
+    }
+  });
+
+  testWidgets('CupertinoListSection.separatorColor defaults CupertinoColors.separator', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Center(
+          child: CupertinoListSection(
+            children: const <Widget>[
+              CupertinoListTile(title: Text('CupertinoListTile')),
+              CupertinoListTile(title: Text('CupertinoListTile')),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    final BuildContext context = tester.element(find.byType(CupertinoListSection));
+    final Column childrenColumn = tester.widget(find.byType(Column).at(1));
+    for (final Widget e in childrenColumn.children) {
+      if (e is Container) {
+        expect(e.color, CupertinoColors.separator.resolveFrom(context));
+      }
+    }
+  });
 }