Add assert for index parameter in IndexedStack. (#167757)

<!--
Thanks for filing a pull request!
Reviewers are typically assigned within a week of filing a request.
To learn more about code review, see our documentation on Tree Hygiene:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
-->

Closes https://github.com/flutter/flutter/issues/165840 by adding an
assert with a more user-friendly message to the IndexedStack.

## 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].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

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

@@ -4265,7 +4265,13 @@ class _RawIndexedStack extends Stack {
     StackFit sizing = StackFit.loose,
     this.index = 0,
     super.children,
-  }) : super(fit: sizing);
+  }) : assert(
+         index == null ||
+             (index == 0 && children.length == 0) ||
+             (index >= 0 && index < children.length),
+         'The index must be null or within the range of children.',
+       ),
+       super(fit: sizing);
 
   /// The index of the child to show.
   final int? index;

packages/flutter/test/widgets/stack_test.dart

@@ -1001,6 +1001,51 @@ void main() {
       'BoxConstraints(2.0<=w<=3.0, 5.0<=h<=7.0)',
     ]);
   });
+
+  testWidgets('IndexedStack does not assert with the default parameters', (
+    WidgetTester tester,
+  ) async {
+    await tester.pumpWidget(
+      const Directionality(textDirection: TextDirection.ltr, child: IndexedStack()),
+    );
+
+    expect(tester.takeException(), isNull);
+  });
+
+  testWidgets('IndexedStack does not assert when index is null', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: IndexedStack(index: null, children: <Widget>[SizedBox.shrink(), SizedBox.shrink()]),
+      ),
+    );
+
+    expect(tester.takeException(), isNull);
+  });
+
+  testWidgets('IndexedStack asserts when index is negative', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: IndexedStack(index: -1, children: <Widget>[SizedBox.shrink(), SizedBox.shrink()]),
+      ),
+    );
+
+    expect(tester.takeException(), isA<AssertionError>());
+  });
+
+  testWidgets('IndexedStack asserts when index is not in children range', (
+    WidgetTester tester,
+  ) async {
+    await tester.pumpWidget(
+      const Directionality(
+        textDirection: TextDirection.ltr,
+        child: IndexedStack(index: 2, children: <Widget>[SizedBox.shrink(), SizedBox.shrink()]),
+      ),
+    );
+
+    expect(tester.takeException(), isA<AssertionError>());
+  });
 }
 
 class _ShowVisibility extends StatelessWidget {