Remove bogus code in ContainerParentDataMixin.detach (#37479)

packages/flutter/lib/src/rendering/object.dart

@@ -2834,21 +2834,9 @@ mixin ContainerParentDataMixin<ChildType extends RenderObject> on ParentData {
   /// Clear the sibling pointers.
   @override
   void detach() {
+    assert(previousSibling == null, 'Pointers to siblings must be nulled before detaching ParentData.');
+    assert(nextSibling == null, 'Pointers to siblings must be nulled before detaching ParentData.');
     super.detach();
-    if (previousSibling != null) {
-      final ContainerParentDataMixin<ChildType> previousSiblingParentData = previousSibling.parentData;
-      assert(previousSibling != this);
-      assert(previousSiblingParentData.nextSibling == this);
-      previousSiblingParentData.nextSibling = nextSibling;
-    }
-    if (nextSibling != null) {
-      final ContainerParentDataMixin<ChildType> nextSiblingParentData = nextSibling.parentData;
-      assert(nextSibling != this);
-      assert(nextSiblingParentData.previousSibling == this);
-      nextSiblingParentData.previousSibling = previousSibling;
-    }
-    previousSibling = null;
-    nextSibling = null;
   }
 }
 

packages/flutter/test/rendering/object_test.dart

@@ -81,8 +81,27 @@ void main() {
       ),
     );
   });
+
+  test('ContainerParentDataMixin requires nulled out pointers to siblings before detach', () {
+    expect(() => TestParentData().detach(), isNot(throwsAssertionError));
+
+    final TestParentData data1 = TestParentData()
+      ..nextSibling = RenderOpacity()
+      ..previousSibling = RenderOpacity();
+    expect(() => data1.detach(), throwsAssertionError);
+
+    final TestParentData data2 = TestParentData()
+      ..previousSibling = RenderOpacity();
+    expect(() => data2.detach(), throwsAssertionError);
+
+    final TestParentData data3 = TestParentData()
+      ..nextSibling = RenderOpacity();
+    expect(() => data3.detach(), throwsAssertionError);
+  });
 }
 
+class TestParentData extends ParentData with ContainerParentDataMixin<RenderBox> { }
+
 class TestRenderObject extends RenderObject {
   @override
   void debugAssertDoesMeetConstraints() { }