Minor adjustments on 2D APIs (#131358)

These tweaks came from https://github.com/flutter/packages/pull/4536

- The TwoDimensionalChildBuilderDelegate asserts that maxXIndex and maxYIndex are null or >= 0
- The TwoDimensionalChildDelegate setter in RenderTwoDimensionalViewport has a covariant to allow type safety for subclasses of RenderTwoDimensionalViewport implementing with other subclasses of TwoDimensionalChildDelegate

I'd like to cherry pick this so https://github.com/flutter/packages/pull/4536 will not have to wait for it to reach stable.

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

@@ -933,7 +933,9 @@ class TwoDimensionalChildBuilderDelegate extends TwoDimensionalChildDelegate {
     required this.builder,
     int? maxXIndex,
     int? maxYIndex,
-  }) : _maxYIndex = maxYIndex,
+  }) : assert(maxYIndex == null || maxYIndex >= 0),
+       assert(maxXIndex == null || maxXIndex >= 0),
+       _maxYIndex = maxYIndex,
        _maxXIndex = maxXIndex;
 
   /// Called to build children on demand.
@@ -974,6 +976,8 @@ class TwoDimensionalChildBuilderDelegate extends TwoDimensionalChildDelegate {
   /// [TwoDimensionalViewport] subclass to learn how this value is applied in
   /// the specific use case.
   ///
+  /// If not null, the value must be non-negative.
+  ///
   /// If the value changes, the delegate will call [notifyListeners]. This
   /// informs the [RenderTwoDimensionalViewport] that any cached information
   /// from the delegate is invalid.
@@ -993,6 +997,7 @@ class TwoDimensionalChildBuilderDelegate extends TwoDimensionalChildDelegate {
     if (value == maxXIndex) {
       return;
     }
+    assert(value == null || value >= 0);
     _maxXIndex = value;
     notifyListeners();
   }
@@ -1015,6 +1020,7 @@ class TwoDimensionalChildBuilderDelegate extends TwoDimensionalChildDelegate {
     if (maxYIndex == value) {
       return;
     }
+    assert(value == null || value >= 0);
     _maxYIndex = value;
     notifyListeners();
   }

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

@@ -612,7 +612,7 @@ abstract class RenderTwoDimensionalViewport extends RenderBox implements RenderA
   /// Supplies children for layout in the viewport.
   TwoDimensionalChildDelegate get delegate => _delegate;
   TwoDimensionalChildDelegate _delegate;
-  set delegate(TwoDimensionalChildDelegate value) {
+  set delegate(covariant TwoDimensionalChildDelegate value) {
     if (_delegate == value) {
       return;
     }

packages/flutter/test/widgets/two_dimensional_viewport_test.dart

@@ -111,6 +111,78 @@ void main() {
         );
       }, variant: TargetPlatformVariant.all());
 
+      test('maxXIndex and maxYIndex assertions', () {
+        final TwoDimensionalChildBuilderDelegate delegate = TwoDimensionalChildBuilderDelegate(
+          maxXIndex: 0,
+          maxYIndex: 0,
+          builder: (BuildContext context, ChildVicinity vicinity) {
+            return const SizedBox.shrink();
+          }
+        );
+        // Update
+        expect(
+          () {
+            delegate.maxXIndex = -1;
+          },
+          throwsA(
+            isA<AssertionError>().having(
+              (AssertionError error) => error.toString(),
+              'description',
+              contains('value == null || value >= 0'),
+            ),
+          ),
+        );
+        expect(
+          () {
+            delegate.maxYIndex = -1;
+          },
+          throwsA(
+            isA<AssertionError>().having(
+              (AssertionError error) => error.toString(),
+              'description',
+              contains('value == null || value >= 0'),
+            ),
+          ),
+        );
+        // Constructor
+        expect(
+          () {
+            TwoDimensionalChildBuilderDelegate(
+              maxXIndex: -1,
+              maxYIndex: 0,
+              builder: (BuildContext context, ChildVicinity vicinity) {
+                return const SizedBox.shrink();
+              }
+            );
+          },
+          throwsA(
+            isA<AssertionError>().having(
+              (AssertionError error) => error.toString(),
+              'description',
+              contains('maxXIndex == null || maxXIndex >= 0'),
+            ),
+          ),
+        );
+        expect(
+          () {
+            TwoDimensionalChildBuilderDelegate(
+              maxXIndex: 0,
+              maxYIndex: -1,
+              builder: (BuildContext context, ChildVicinity vicinity) {
+                return const SizedBox.shrink();
+              }
+            );
+          },
+          throwsA(
+            isA<AssertionError>().having(
+              (AssertionError error) => error.toString(),
+              'description',
+              contains('maxYIndex == null || maxYIndex >= 0'),
+            ),
+          ),
+        );
+      });
+
       testWidgets('throws an error when builder throws', (WidgetTester tester) async {
         final List<Object> exceptions = <Object>[];
         final FlutterExceptionHandler? oldHandler = FlutterError.onError;
@@ -1822,3 +1894,28 @@ Future<void> restoreScrollAndVerify(WidgetTester tester) async {
     100.0,
   );
 }
+
+// Validates covariant through analysis.
+mixin _SomeDelegateMixin on TwoDimensionalChildDelegate {}
+
+class _SomeRenderTwoDimensionalViewport extends RenderTwoDimensionalViewport { // ignore: unused_element
+  _SomeRenderTwoDimensionalViewport({
+    required super.horizontalOffset,
+    required super.horizontalAxisDirection,
+    required super.verticalOffset,
+    required super.verticalAxisDirection,
+    required _SomeDelegateMixin super.delegate,
+    required super.mainAxis,
+    required super.childManager,
+  });
+
+  @override
+  _SomeDelegateMixin get delegate => super.delegate as _SomeDelegateMixin;
+  @override
+  set delegate(_SomeDelegateMixin value) { // Analysis would fail without covariant
+    super.delegate = value;
+  }
+
+  @override
+  void layoutChildSequence() {}
+}