Remove `_debugWillReattachChildren` assertions from `_TableElement` (#34202)

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

@@ -250,13 +250,9 @@ class _TableElement extends RenderObjectElement {
 
   List<_TableElementRow> _children = const<_TableElementRow>[];
 
-  bool _debugWillReattachChildren = false;
-
   @override
   void mount(Element parent, dynamic newSlot) {
     super.mount(parent, newSlot);
-    assert(!_debugWillReattachChildren);
-    assert(() { _debugWillReattachChildren = true; return true; }());
     _children = widget.children.map<_TableElementRow>((TableRow row) {
       return _TableElementRow(
         key: row.key,
@@ -266,32 +262,20 @@ class _TableElement extends RenderObjectElement {
         }).toList(growable: false),
       );
     }).toList(growable: false);
-    assert(() { _debugWillReattachChildren = false; return true; }());
     _updateRenderObjectChildren();
   }
 
   @override
   void insertChildRenderObject(RenderObject child, Element slot) {
-    assert(_debugWillReattachChildren);
     renderObject.setupParentData(child);
   }
 
   @override
   void moveChildRenderObject(RenderObject child, dynamic slot) {
-    assert(_debugWillReattachChildren);
   }
 
   @override
   void removeChildRenderObject(RenderObject child) {
-    assert(() {
-      if (_debugWillReattachChildren)
-        return true;
-      for (Element forgottenChild in _forgottenChildren) {
-        if (forgottenChild.renderObject == child)
-          return true;
-      }
-      return false;
-    }());
     final TableCellParentData childParentData = child.parentData;
     renderObject.setChild(childParentData.x, childParentData.y, null);
   }
@@ -300,8 +284,6 @@ class _TableElement extends RenderObjectElement {
 
   @override
   void update(Table newWidget) {
-    assert(!_debugWillReattachChildren);
-    assert(() { _debugWillReattachChildren = true; return true; }());
     final Map<LocalKey, List<Element>> oldKeyedRows = <LocalKey, List<Element>>{};
     for (_TableElementRow row in _children) {
       if (row.key != null) {
@@ -330,7 +312,7 @@ class _TableElement extends RenderObjectElement {
       updateChildren(oldUnkeyedRows.current.children, const <Widget>[], forgottenChildren: _forgottenChildren);
     for (List<Element> oldChildren in oldKeyedRows.values.where((List<Element> list) => !taken.contains(list)))
       updateChildren(oldChildren, const <Widget>[], forgottenChildren: _forgottenChildren);
-    assert(() { _debugWillReattachChildren = false; return true; }());
+
     _children = newChildren;
     _updateRenderObjectChildren();
     _forgottenChildren.clear();

packages/flutter/test/widgets/table_test.dart

@@ -19,6 +19,24 @@ class TestStatefulWidgetState extends State<TestStatefulWidget> {
   Widget build(BuildContext context) => Container();
 }
 
+class TestChildWidget extends StatefulWidget {
+  const TestChildWidget({ Key key }) : super(key: key);
+
+  @override
+  TestChildState createState() => TestChildState();
+}
+
+class TestChildState extends State<TestChildWidget> {
+  bool toggle = true;
+
+  void toggleMe() {
+    setState(() { toggle = !toggle; });
+  }
+
+  @override
+  Widget build(BuildContext context) => toggle ? const SizedBox() : const Text('CRASHHH');
+}
+
 void main() {
   testWidgets('Table widget - empty', (WidgetTester tester) async {
     await tester.pumpWidget(
@@ -855,5 +873,32 @@ void main() {
     );
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/31473.
+  testWidgets(
+    'Does not crash if a child RenderObject is replaced by another RenderObject of a different type',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        Directionality(
+          textDirection: TextDirection.ltr,
+          child: Table(children: const <TableRow>[TableRow(children: <Widget>[TestChildWidget()])]),
+        ),
+      );
+      expect(find.text('CRASHHH'), findsNothing);
+
+      final TestChildState state = tester.state(find.byType(TestChildWidget));
+      state.toggleMe();
+
+      await tester.pumpWidget(
+        Directionality(
+          textDirection: TextDirection.ltr,
+          child: Table(children: const <TableRow>[TableRow(children: <Widget>[TestChildWidget()])]),
+        ),
+      );
+
+      // Should not crash.
+      expect(find.text('CRASHHH'), findsOneWidget);
+    }
+  );
+
   // TODO(ianh): Test handling of TableCell object
 }