[MergeableMaterial] adds dividerColor property (#59481)

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

@@ -107,6 +107,7 @@ class MergeableMaterial extends StatefulWidget {
     this.elevation = 2,
     this.hasDividers = false,
     this.children = const <MergeableMaterialItem>[],
+    this.dividerColor,
   }) : super(key: key);
 
   /// The children of the [MergeableMaterial].
@@ -128,6 +129,12 @@ class MergeableMaterial extends StatefulWidget {
   /// Whether connected pieces of [MaterialSlice] have dividers between them.
   final bool hasDividers;
 
+  /// Defines color used for dividers if [hasDividers] is true.
+  ///
+  /// If `dividerColor` is null, then [DividerThemeData.color] is used. If that
+  /// is null, then [ThemeData.dividerColor] is used.
+  final Color dividerColor;
+
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
@@ -562,6 +569,7 @@ class _MergeableMaterialState extends State<MergeableMaterial> with TickerProvid
           final BorderSide divider = Divider.createBorderSide(
             context,
             width: 0.5, // TODO(ianh): This probably looks terrible when the dpr isn't a power of two.
+            color: widget.dividerColor,
           );
 
           if (i == 0) {

packages/flutter/test/material/mergeable_material_test.dart

@@ -1166,4 +1166,41 @@ void main() {
     expect(isDivider(boxes[offset + 3], false, true), isTrue);
     expect(isDivider(boxes[offset + 4], true, false), isTrue);
   });
+
+  testWidgets('MergeableMaterial respects dividerColor', (WidgetTester tester) async {
+    const Color dividerColor = Colors.red;
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Scaffold(
+          body: SingleChildScrollView(
+            child: MergeableMaterial(
+              hasDividers: true,
+              dividerColor: dividerColor,
+              children: <MergeableMaterialItem>[
+                MaterialSlice(
+                  key: ValueKey<String>('A'),
+                  child: SizedBox(
+                    width: 100.0,
+                    height: 100.0,
+                  ),
+                ),
+                MaterialSlice(
+                  key: ValueKey<String>('B'),
+                  child: SizedBox(
+                    width: 100.0,
+                    height: 100.0,
+                  ),
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    final DecoratedBox decoratedBox = tester.widget(find.byType(DecoratedBox).last);
+    final BoxDecoration decoration = decoratedBox.decoration as BoxDecoration;
+    // Since we are getting the last DecoratedBox, it will have a Border.top.
+    expect(decoration.border.top.color, dividerColor);
+  });
 }