This is a proof of concept for renaming SlottedMultiChildRenderObjectWidgetMixin to SlottedMultiChildRenderObjectWidget and making it a concrete class.
I also made SlottedContainerRenderObjectMixin generic instead of being specialized to RenderBox.
I don't think this is something we can easily automigrate, but we may not need to, I don't know how common this is...
This PR does a couple of things!
https://user-images.githubusercontent.com/16964204/231897483-416287f9-50ce-468d-a714-2a4bc0f2e011.mov

Fixes#20819Fixes#41910Fixes#121419
### Adds ScrollController.onAttach and ScrollController.onDetach
This resolves a long held pain point for developers. When using a scroll controller, there is not scroll position until the scrollable widget is built, and almost all methods of notification are only triggered when scrolling happens. Adding these two methods will help developers gain access to the scroll position when it is created. A common workaround for this was using a post frame callback to access controller.position after the first frame, but this is ripe for issues such as having multiple positions attached to the controller, or the scrollable no longer existing after that post frame callback. I think this can also be helpful for folks to debug cases when the scroll controller has multiple positions attached.
In particular, this also resolves this commented case: https://github.com/flutter/flutter/issues/20819#issuecomment-417784218
The isScrollingNotifier is hard for developers to access.
### Docs & samples
I was surprised we did not have samples on scroll notification or scroll controller, so I overhauled it and added a lot of docs on all the different ways to access scrolling information, when it is available and how they differ.
* fix: gets removedItem instead of its index
add: sliver_animated_list.0_test.dart
* fix: sliver_animated_list.0_test.dart
* fix: pr comments
* fix test import
Co-authored-by: Taha Tesser <tessertaha@gmail.com>
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Co-authored-by: Taha Tesser <tessertaha@gmail.com>
* Add support for image insertion on Android
* Fix checks
* Use proper Dart syntax on snippet
* Specify type annotation on list
* Fix nits, add some asserts, and improve example code
* Add missing import
* Fix nullsafety error
* Fix nullsafety error
* Remove reference to contentCommitMimeTypes in docs
* Fix nits
* Fix warnings and import
* Add test for content commit in editable_text_test.dart
* Check that URIs are equal in test
* Fix nits and rename functions / classes to be more self-explanatory
* Fix failing debugFillProperties tests
* Add empty implementation to `insertContent` in TextInputClient
* Tweak documentation slightly
* Improve docs for contentInsertionMimeTypes and fix assert
* Rework contentInsertionMimeType asserts
* Add test for onContentInserted example
* Switch implementation to a configuration class for more granularity in setting mime types
* Fix nits
* Improve docs and fix doc tests
* Fix more nits (LongCatIsLooong)
* Fix failing tests
* Make parameters (guaranteed by platform to be non-nullable) non-nullable
* Fix analysis issues
* Add MenuMenuAcceleratorLabel to support accelerators.
* Review Changes
* Review Changed
* Fix default label builder to use characters
* Remove golden test that shouldn't have been there.