*Use `super.key` instead of manually passing the `Key` parameter using super(key: key) in the constructors.*
Since if you create a widget the new default will use `super.key` instead of `Key? key : super(key: key)` this small change is to maintain the consistency, it has no semantic change
also there are some other places that might need to be updated:

this file for example generate l10n project and it has all the dart code as String, it might have tests that validate the output somewhere that I might miss, also there are some other places like the `_Segment` class where it require `ValueKey` instead if `Key` so I didn't update them (even though it's possible)
The documentation for using `findChildIndexCallback` recommends using `indexOf`, but that causes [this line](05259ca938/packages/flutter/lib/src/rendering/sliver_multi_box_adaptor.dart (L259)) to throw in debug mode, and when using `SliverList`, it breaks the render.
This PR changes the usage to check if the index is not negative before using it, and changes to return `null` instead if the child wasn't able to be found.
There's the related issue #107123, but this doesn't actually fix it.
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This PR has been updated to add the snippets that were used in the `findChildIndexCallback` comment as examples with proper tests, as well as updating the comment to reference the new examples.
This does a cleanup of the examples, removing all of the "section" markers and extra comments that we don't need anymore now that the samples are no longer in the source code. It also removes the --template arguments from the {@tool dartpad} and {@tool sample} directives, since those are no longer used. It converts two examples that I discovered were still embedded into linked examples in the examples folder.
I didn't delete the templates from the snippets config folder yet, because there are still embedded samples in the dart:ui package from the engine that use them. Once dart:ui no longer uses the templates, they can be removed.
I bumped the version of the snippets package to pick up a change that allows removal of the --template argument.
This extracts the sample code out from the API doc comments, and places them in separate files on disk, allowing running of the examples locally, testing them, and building of slightly larger examples.