Test SliverMainAxisGroup offstage child (#159406)

```dart
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  testWidgets('my test', (WidgetTester tester) async {
    await tester.pumpWidget(
      MaterialApp(
        home: LayoutBuilder(
          builder: (context, view) {
            return CustomScrollView(
              slivers: [
                SliverMainAxisGroup(
                  slivers: [
                    SliverToBoxAdapter(child: SizedBox(height: view.maxHeight)),
                    const SliverToBoxAdapter(child: Text('1')),
                  ],
                )
              ],
            );
          },
        ),
      ),
    );
    expect(find.text('1'), findsNothing);
  });
}
```
The above test should have passed, but it failed. This PR fixes the
issue.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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[Features we expect every widget to implement]:
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[CLA]: https://cla.developers.google.com/
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[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

examples/api/test/widgets/sliver/sliver_main_axis_group.0_test.dart

@@ -19,7 +19,16 @@ void main() {
 
     final RenderSliverPersistentHeader renderAppBar = tester.renderObject<RenderSliverPersistentHeader>(find.byType(SliverAppBar));
     final RenderSliverList renderSliverList = tester.renderObject<RenderSliverList>(find.byType(SliverList));
-    final RenderSliverToBoxAdapter renderSliverAdapter = tester.renderObject<RenderSliverToBoxAdapter>(find.byType(SliverToBoxAdapter));
+    final RenderSliverToBoxAdapter renderSliverAdapter = tester.renderObject<RenderSliverToBoxAdapter>(
+      find.descendant(
+        of: find.byType(SliverMainAxisGroup),
+        matching: find.byType(
+          SliverToBoxAdapter,
+          skipOffstage: false,
+        ),
+        skipOffstage: false,
+      ),
+    );
 
     // renderAppBar, renderSliverList, and renderSliverAdapter1 are part of the same sliver group.
     expect(renderAppBar.geometry!.scrollExtent, equals(70.0));

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

@@ -1741,8 +1741,23 @@ class SliverMainAxisGroup extends MultiChildRenderObjectWidget {
     required List<Widget> slivers,
   }) : super(children: slivers);
 
+  @override
+  MultiChildRenderObjectElement createElement() => _SliverMainAxisGroupElement(this);
+
   @override
   RenderSliverMainAxisGroup createRenderObject(BuildContext context) {
     return RenderSliverMainAxisGroup();
   }
 }
+
+class _SliverMainAxisGroupElement extends MultiChildRenderObjectElement {
+  _SliverMainAxisGroupElement(SliverMainAxisGroup super.widget);
+
+  @override
+  void debugVisitOnstageChildren(ElementVisitor visitor) {
+    children.where((Element e) {
+      final RenderSliver renderSliver = e.renderObject! as RenderSliver;
+      return renderSliver.geometry!.visible;
+    }).forEach(visitor);
+  }
+}

packages/flutter/test/widgets/editable_text_test.dart

@@ -16960,7 +16960,7 @@ void main() {
       ),
     );
     await tester.pumpWidget(widget);
-    await tester.showKeyboard(find.byType(EditableText));
+    await tester.showKeyboard(find.byType(EditableText, skipOffstage: false));
     await tester.pumpAndSettle();
     expect(scrollController.offset, 75.0);
   });

packages/flutter/test/widgets/sliver_main_axis_group_test.dart

@@ -198,7 +198,7 @@ void main() {
       const Rect.fromLTRB(0.0, 0.0, 200.0, 600.0),
     );
 
-    final List<RenderSliverList> renderSlivers = tester.renderObjectList<RenderSliverList>(find.byType(SliverList)).toList();
+    final List<RenderSliverList> renderSlivers = tester.renderObjectList<RenderSliverList>(find.byType(SliverList, skipOffstage: false)).toList();
     final RenderSliverList first = renderSlivers[0];
     final RenderSliverList second = renderSlivers[1];
 
@@ -268,7 +268,7 @@ void main() {
       const Rect.fromLTRB(100.0, 0.0, 300.0, 600.0),
     );
 
-    final List<RenderSliverList> renderSlivers = tester.renderObjectList<RenderSliverList>(find.byType(SliverList)).toList();
+    final List<RenderSliverList> renderSlivers = tester.renderObjectList<RenderSliverList>(find.byType(SliverList, skipOffstage: false)).toList();
     final RenderSliverList first = renderSlivers[0];
     final RenderSliverList second = renderSlivers[1];
 
@@ -598,7 +598,7 @@ void main() {
     await tester.pumpAndSettle();
 
     // At a scroll offset of 630, a normal scrolling header should be out of view.
-    final RenderSliverPersistentHeader renderHeader = tester.renderObject(find.byType(SliverPersistentHeader)) as RenderSliverPersistentHeader;
+    final RenderSliverPersistentHeader renderHeader = tester.renderObject(find.byType(SliverPersistentHeader, skipOffstage: false)) as RenderSliverPersistentHeader;
     expect(renderHeader.constraints.scrollOffset, equals(630));
     expect(renderHeader.geometry!.layoutExtent, equals(0.0));
   });
@@ -855,6 +855,20 @@ void main() {
 
     expect(tester.getTopLeft(find.byKey(firstTextFieldKey)), const Offset(0, 60));
   });
+
+  testWidgets('SliverMainAxisGroup offstage child', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      _buildSliverMainAxisGroup(
+        viewportHeight: 300,
+        slivers: <Widget>[
+          const SliverToBoxAdapter(child: SizedBox(height: 300)),
+          const SliverToBoxAdapter(child: SizedBox(height: 100, child: Text('1'))),
+        ]
+      )
+    );
+    expect(find.text('1'), findsNothing);
+    expect(find.text('1', skipOffstage: false), findsOneWidget);
+  });
 }
 
 Widget _buildSliverList({