## 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after null safety has been made the default, these comments were largely fragile, in that it was easy for them to get out of date, and not be accurate anymore anyhow.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.

* Remove unnecessary null checks in flutter/animation

* Remove unnecessary null checks in flutter/semantics

* Remove unnecessary null checks in flutter/scheduler

* Use `curly_braces_in_flow_control_structures` for `animation`

* Use `curly_braces_in_flow_control_structures` for `cupertino`

* fix comments

Update samples to use repo analysis options, Fix sample templates and a ton of analyzer issues (#77868)

* migrate animation to nullsafety

* nullable tween

* fix generic type issue

* address review comment

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

* Android 10 zoom transition

Fix the spelling errors in the dartdocs for the framework.

There are no functionality changes here, just documentation fixes.

## Description

Also update the existing dart files with missing licenses.

Without the fix, we'll emit the following error message
```
License headers cannot be found at the beginning of the following files.

/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/lib/src/animation/tween_sequence.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/material/raw_material_button_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/async_lifecycle_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/sliver_constraints_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/app_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/test_border.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/physical_model_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/inherited_model.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter_tools/lib/src/base/user_messages.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter_tools/test/src/pubspec_schema.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter_tools/test/ios/simulators_test.dart
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
```

## Related Issues

Fixes https://github.com/flutter/flutter/issues/28368

## Checklist

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- [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
- [x] My PR includes tests for *all* changed/updated/fixed behaviors (See [Test Coverage]).
- [x] All existing and new tests are passing.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] The analyzer (`flutter analyze --flutter-repo`) does not report any problems on my PR.
- [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I am willing to follow-up on review comments in a timely manner.

## Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

- [ ] Yes, this is a breaking change (Please read [Handling breaking changes]).
- [x] No, this is *not* a breaking change.

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This reverts commit 4ec8883b.

This converts existing ## Sample code samples to {@tool sample}...{@end-tool} form.

Also:
1. Fixed a minor bug in analyze-sample-code.dart
2. Made the snippet tool only insert descriptions if the description is non-empty.
3. Moved the Card diagram to before the code sample.

This reverts commit 76a1abd4.

This reverts commit 949cfcf1.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* lint unnecessary_new on samples

* fix tests

Make it possible to define an animation by stringing together a sequence of tweens.