This PR adds the linux and windows target platform enum values, along with automatically setting the defaultTargetPlatform to the appropriate value on those platforms.

Fixes #31366

* 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

This PR adds TargetPlatform.macOS to the TargetPlatform enum. This allows us to begin implementation of some adaptive UI based on which target platform is desired.

I haven't updated the tests here, that will come in a follow-up PR.

* make BuildContext.{ancestorStateOfType,ancestorRenderObjectOfType,rootAncestorStateOfType} generic

* make BuildContext.inheritFromWidgetOfExactType generic

* make BuildContext.ancestorInheritedElementForWidgetOfExactType generic

* make BuildContext.ancestorWidgetOfExactType generic

* fix snippet

* bump scoped_model on temp version

* update names

* Revert "bump scoped_model on temp version"

This reverts commit d1fcbba028cdb07f44738d1652391692d1ea5ec0.

* address review comments

* fix ci

* address review comments

* repeat the deprecation notice

* fix uppercase

* use of recommanded deprecation syntax

* address review comment

## 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

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes (`[x]`). This will ensure a smooth and quick review process.

- [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.

<!-- Links -->
[issue database]: https://github.com/flutter/flutter/issues
[Contributor Guide]: https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Test Coverage]: https://github.com/flutter/flutter/wiki/Test-coverage-for-package%3Aflutter
[Flutter Style Guide]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[Handling breaking changes]: https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

Turns out we have many immutable classes.

Fixes #6892

Mostly related to semantics and scrolling.

This method seemed overly specific to the needs of one particular
subclass. This patch duplicates some code but makes the API conceptually
cleaner.

Fixes #8267

Fixes #7734

Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

All the clients have migrated to Scrollable2.

This patch makes a number of changes to how you can configure a
Scrollable2:

 - The ScrollPhysics is now responsible for creating the ScrollPosition.
   You can override the ScrollPhysics by supplying a `physics` argument
   to `Scrollable`, and the new physics you supply will be applied to
   the default physics inherited from the ScrollBehavior.

 - This patch removes the ScrollPosition/AbsoluteScrollPosition split as
   all clients were operating in pixels anyway and the split made the
   code very difficult to follow.

 - ScrollPosition no longer depends directly on Scrollable2State.
   Instead, it depends on an abstract interface that Scrollable2State
   implements. This change has two benefits:

    a) It removes the circular dependency between ScrollPosition and
       Scrollable2State, which lets us split the code for these classes
       (and several other classes that got wrapped up in that cycle) into
       separate libraries for easier maintenance.

    b) ScrollPosition is no longer bound to Scrollable2, which means you
       could use the behavior machinery to drive other sorts of widgets.
       For example, we could use it to drive Scrollabe1 if we wanted.

This patch adds `@checked` everywhere is needed to remove the
`strong_mode_invalid_method_override` strong mode error.

* Added OverscrollIndicatorEdge et al

* RefreshIndicator only clamps its scrollable edge

* added a test

* Updated the test

* fixed lint-os

* fixed a typo

* Scrollable should restore its viewport dimensions when it reappears

* removed an accidental commit

* updated per review feedback

Changes in this patch:
- iOS now uses a different scrollDrag constant than Android.
   - ScrollConfigurationDelegate now knows about target platforms.
   - ScrollBehaviors now know about target platforms.
   - RawInputLine now has to be told what platform it's targetting.
   - PageableList now has a concept of target platform.
- make debugPrintStack filter its stack.
   - move debugPrintStack to `assertions.dart`.
- add support for limiting the number of frames to debugPrintStack.
- make defaultTargetPlatform default to android in test environments.
- remove OverscrollStyle and MaterialApp's overscrollStyle argument. You
  can now control the overscroll style using Theme.platform.
- the default scroll configuration is now private to avoid people
  relying on the defaultTargetPlatform getter in their subclasses (since
  they really should use Theme.of(context).platform).
- fix some typos I noticed in some tests.
- added a test for flinging scrollables, that checks that the behavior
  differs on the two target platforms.
- made flingFrom and fling in the test API pump the frames.
- added more docs to the test API.
- made the TestAsyncUtils.guard() method report uncaught errors to help
  debug errors when using that API.

Also, add more dartdoc to widgets.dart.