## Description

This removes all of the comments that are of the form "so-and-so (must not be null|can ?not be null|must be non-null)" from the cases where those values are defines as non-nullable values.

This PR removes them from the painting and rendering libraries.

This was done by hand, since it really didn't lend itself to scripting, so it needs to be more than just spot-checked, I think. I was careful to leave any comment that referred to parameters that were nullable, but I may have missed some.

In addition to being no longer relevant after null safety has been made the default, these comments were largely fragile, in that it was easy for them to get out of date, and not be accurate anymore anyhow.

This did create a number of constructor comments which basically say "Creates a [Foo].", but I don't really know how to avoid that in a large scale change, since there's not much you can really say in a lot of cases.  I think we might consider some leniency for constructors to the "Comment must be meaningful" style guidance (which we de facto have already, since there are a bunch of these).

## Related PRs
- https://github.com/flutter/flutter/pull/134984
- https://github.com/flutter/flutter/pull/134991
- https://github.com/flutter/flutter/pull/134992
- https://github.com/flutter/flutter/pull/134994

## Tests
 - Documentation only change.

migration guide update https://github.com/flutter/website/pull/9124

fixes the concerns in https://github.com/flutter/flutter/pull/120619

Reimplements what we reverted here: #125233.

Going to make some changes to the implementation so I'll revert in the meantime.

Reverts flutter/flutter#124337

Sliver Constrained Cross Axis

Fixes IgnorePointer and AbsorbPointer to only block user interactions…

* Use `curly_braces_in_flow_control_structures` for `rendering`

* fix comments

* fix dangling

* Make sure Opacity widgets/layers do not drop the offset

This reverts commit 0d0f7a4f.

* apply upcoming lint cast_nullable_to_non_nullable

* make RenderObjectElement.renderObject non-nullable

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

This reverts commit c235998a.