refactor: use skipOffstage to test children in IndexedStack

Author: okaryo

test/lazy_load_indexed_stack_test.dart

@@ -24,32 +24,11 @@ void main() {
     final elementState = element.state as State<LazyLoadIndexedStack>;
     expect(elementState.widget, equals(lazyLoadIndexedStack));
 
-    final indexedStack = find.byType(IndexedStack);
-    expect(indexedStack, findsOneWidget);
-
-    final IndexedStack indexedStackWidget = tester.widget(indexedStack) as IndexedStack;
-    final children = indexedStackWidget.children;
-
-    // page1 is loaded.
-    expect(find.text('page1'), findsOneWidget);
-
-    // page2 is loaded.
-    bool hasPage2 = children.any((Widget widget) {
-      return widget is Center && widget.child is Text && (widget.child as Text).data == 'page2';
-    });
-    expect(hasPage2, isTrue);
-
-    // page3 is not loaded.
-    expect(find.text('page3'), findsNothing);
-
-    // page4 is loaded.
-    bool hasPage4 = children.any((Widget widget) {
-      return widget is Center && widget.child is Text && (widget.child as Text).data == 'page4';
-    });
-    expect(hasPage4, isTrue);
-
-    // page5 is not loaded.
-    expect(find.text('page5'), findsNothing);
+    expect(find.text('page1', skipOffstage: false), findsOneWidget);
+    expect(find.text('page2', skipOffstage: false), findsOneWidget);
+    expect(find.text('page3', skipOffstage: false), findsNothing);
+    expect(find.text('page4', skipOffstage: false), findsOneWidget);
+    expect(find.text('page5', skipOffstage: false), findsNothing);
   });
 }