Reverts flutter/flutter#134953

Several failures on CI

Documentation improvements

Bump lower Dart SDK constraints to 3.0 & add class modifiers

* Add ListenableBuilder with examples

* Add tests

* Add tests

* Fix Test

* Change AnimatedBuilder to be a subclass of ListenableBuilder

* Disallow dispose during listener callback

* addressing comment

* add comments to code

* Addressing comments

* fix test

This reverts part of the change made in #103456 to expose a debug check for subclasses of ChangeNotifier to avoid code duplication. Instead of making debugAssertNotDisposed a public instance function, it is now a public static function. It makes it harder to call, slightly, but it means that everyone who implemented ChangeNotifier instead of extending it doesn't get broken.

Use `curly_braces_in_flow_control_structures` for `foundation`, `gestures`, `painting`, `physics` (#104610)

* Use `curly_braces_in_flow_control_structures` for `foundation`

* Use `curly_braces_in_flow_control_structures` for `gestures`

* Use `curly_braces_in_flow_control_structures` for `painting`

* Use `curly_braces_in_flow_control_structures` for `physics`

* fix comments

* remove trailing space

* fix TODO style

This adds a ShortcutsRegistry for ShortcutActivator to Intent mappings that can be modified from its descendants.

This is so that descendants can make shortcuts dynamically available to a larger portion of the app than just their descendants. This is a precursor needed by the new MenuBar, for instance, so that the menu bar itself can be placed where it likes, but the shortcuts it defines can be in effect for most, if not all, of the UI surface in the app. For example, the "Ctrl-Q" quit binding would need to work even if the focused widget wasn't a child of the MenuBar.

This just provides the shortcut to intent mapping, the actions activated by the intent are described in the context where they make sense. For example, defining a "Ctrl-C" shortcut mapped to a "CopyIntent" should perform different functions if it happens while a TextField has focus vs when a drawing has focus, so those different areas would need to define different actions mapped to "CopyIntent". A hypothetical "QuitIntent" would probably be active for the entire app, so would be mapped in an Actions widget near the top of the hierarchy.

* fix deprecated_new_in_comment_reference for `material` library

in a future version of the SDK, these will be flagged, fix them now

* Update pubspec.yaml

- - When I added notification of key events before processing them as text, it made it so that shortcut key bindings like the spacebar would prevent spaces from being inserted into text fields, which is obviously not desirable (and so that change was reverted). At the same time, we do want to make it possible to override key events so that they can do things like intercept a tab key or arrow keys that change the focus.

This PR changes the behavior of the Shortcuts widget so that if it has a shortcut defined, but no action is bound to the intent, then instead of responding that the key is "handled", it responds as if nothing handled it. This allows the engine to continue to process the key as text entry.

This PR includes:

- Modification of the callback type for key handlers to return a KeyEventResult instead of a bool, so that we can return more information (i.e. the extra state of "stop propagation").
- Modification of the ActionDispatcher.invokeAction contract to require that Action.isEnabled return true before calling it. It will now assert if the action isn't enabled when invokeAction is called. This is to allow optimization of the number of calls to isEnabled, since the shortcuts widget now wants to know if the action was enabled before deciding to either handle the key or to return ignored.
- Modification to ShortcutManager.handleKeypress to return KeyEventResult.ignored for keys which don't have an enabled action associated with them.
- Adds an attribute to DoNothingAction that allows it to mark a key as not handled, even though it does have an action associated with it. This will allow disabling of a shortcut for a subtree.

* Use a LinkedList in ChangeNotifier implementation to take O(N^2) notification time to O(N)

* migrate foundation to nullsafety

* address review comments

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

This updates the Action API in accordance with the design doc for the changes: flutter.dev/go/actions-and-shortcuts-design-revision

Fixes #53276

This adds a sample for InheritedNotifier, and converts a couple of other samples to be DartPad samples. I also added a new sample template stateful_widget_material_ticker, which adds a TickerProviderStateMixin to the state object so that animation controllers can be created there easily.

This PR is a follow up of https://github.com/flutter/flutter/pull/42640
Some changes of that PR includes redundant changes using FlutterError.fromParts constructor even though it's not necessary.

Some minor changes are:

- Remove one unnecessary todo
- Fix indent consistencies

* 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