Override MultiChildRenderObjectElement renderObject getter to provide covariant type (#72530)

packages/flutter/lib/src/widgets/framework.dart

@@ -6223,6 +6223,11 @@ class MultiChildRenderObjectElement extends RenderObjectElement {
   @override
   MultiChildRenderObjectWidget get widget => super.widget as MultiChildRenderObjectWidget;
 
+  @override
+  ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>> get renderObject {
+    return super.renderObject as ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>>;
+  }
+
   /// The current list of children of this element.
   ///
   /// This list is filtered to hide elements that have been forgotten (using
@@ -6238,8 +6243,7 @@ class MultiChildRenderObjectElement extends RenderObjectElement {
 
   @override
   void insertRenderObjectChild(RenderObject child, IndexedSlot<Element?> slot) {
-    final ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>> renderObject =
-      this.renderObject as ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>>;
+    final ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>> renderObject = this.renderObject;
     assert(renderObject.debugValidateChild(child));
     renderObject.insert(child, after: slot.value?.renderObject);
     assert(renderObject == this.renderObject);
@@ -6247,8 +6251,7 @@ class MultiChildRenderObjectElement extends RenderObjectElement {
 
   @override
   void moveRenderObjectChild(RenderObject child, IndexedSlot<Element?> oldSlot, IndexedSlot<Element?> newSlot) {
-    final ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>> renderObject =
-      this.renderObject as ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>>;
+    final ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>> renderObject = this.renderObject;
     assert(child.parent == renderObject);
     renderObject.move(child, after: newSlot.value?.renderObject);
     assert(renderObject == this.renderObject);
@@ -6256,8 +6259,7 @@ class MultiChildRenderObjectElement extends RenderObjectElement {
 
   @override
   void removeRenderObjectChild(RenderObject child, dynamic slot) {
-    final ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>> renderObject =
-      this.renderObject as ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>>;
+    final ContainerRenderObjectMixin<RenderObject, ContainerParentDataMixin<RenderObject>> renderObject = this.renderObject;
     assert(child.parent == renderObject);
     renderObject.remove(child);
     assert(renderObject == this.renderObject);