Fixes https://github.com/flutter/flutter/issues/137696
This will get the tree green again.
The problem is that `CustomSemanticsAction` has static state in it, and todays test seed order makes things unhappy.
*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*
fix some typos
*List which issues are fixed by this PR. You must list at least one issue.*
*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*
## 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 animation, cupertino, foundation, gestures, semantics, and services libraries. Each of them only had a few, so I lumped them together.
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after null safety has been made the default, these comments were largely fragile, in that it was easy for them to get out of date, and not be accurate anymore anyhow.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/134991
- https://github.com/flutter/flutter/pull/134992
- https://github.com/flutter/flutter/pull/134993
- https://github.com/flutter/flutter/pull/134994
## Tests
- Documentation only change.
These methods and/or their docs were recently copied (in #128467 and #128973) from their classes' former shared base class AbstractNode. Their wording was fittingly abstract there, but that abstraction is a bit puzzling for a reader finding them on these more concrete classes and not aware of the AbstractNode history. So make them more concrete, in similar terms to the other methods around them.
Also copy some useful points between corresponding methods on different classes (like that the parent of the root is null), and try to clean up the prose on [RenderObject.depth].
We focus on the more outward-facing parts of the API, letting methods like `redepthChildren` continue to talk generically about "nodes".
* Revert "Revert "Fix text.rich to merge widget span (#113461)" (#121562)"
This reverts commit c8d80163b03f76e0cfde15567d065b6d1903cf8d.
* Fixes tag to not pollute cached semantics configuration in rendering object
* Reland "Adds API in semanticsconfiguration to decide how to merge child semanticsConfigurations (#110730)"
This reverts commit 7549925c8c44dea92c7bc75be676c17b7613f87f.
* makes markNeedsSemanticsUpdate more robust
* address comment
