This PR introduces `AnimationStyle`, it is used to override default
animation curves and durations in several widgets.

fixes [Add the ability to customize MaterialApp theme animation
duration](https://github.com/flutter/flutter/issues/78372)
fixes [Allow customization of showMenu transition animation curves and
duration](https://github.com/flutter/flutter/issues/135638)
fixes [`AnimationStyle.noAnimation` needs to replace `AnimatedTheme`
with just `Theme` in the
`MaterialApp`](https://github.com/flutter/flutter/issues/138618)

Here is an example where popup menu curve and transition duration is
overridden:

```dart
          popUpAnimationStyle: AnimationStyle(
            curve: Easing.emphasizedAccelerate,
            duration: Durations.medium4,
          ),
```

Set `AnimationStyle.noAnimation` to disable animation.
```dart
    return MaterialApp(
      themeAnimationStyle: AnimationStyle.noAnimation,
```

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
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https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[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/
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[breaking change policy]:
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[Discord]: https://github.com/flutter/flutter/wiki/Chat

Reverts flutter/flutter#137945
Initiated by: HansMuller
This change reverts the following previous change:
Original Description:
This PR introduces `AnimationStyle`, it is used to override default animation curves and durations in several widgets.

fixes  [Add the ability to customize MaterialApp theme animation duration](https://github.com/flutter/flutter/issues/78372)
fixes [Allow customization of showMenu transition animation curves and duration](https://github.com/flutter/flutter/issues/135638)

Here is an example where popup menu curve and transition duration is overriden:

```dart
          popUpAnimationStyle: AnimationStyle(
            curve: Easing.emphasizedAccelerate,
            duration: Durations.medium4,
          ),
```

Set `AnimationStyle.noAnimation` to disable animation.
```dart
    return MaterialApp(
      themeAnimationStyle: AnimationStyle.noAnimation,
```

This PR introduces `AnimationStyle`, it is used to override default animation curves and durations in several widgets.

fixes  [Add the ability to customize MaterialApp theme animation duration](https://github.com/flutter/flutter/issues/78372)
fixes [Allow customization of showMenu transition animation curves and duration](https://github.com/flutter/flutter/issues/135638)

Here is an example where popup menu curve and transition duration is overriden:

```dart
          popUpAnimationStyle: AnimationStyle(
            curve: Easing.emphasizedAccelerate,
            duration: Durations.medium4,
          ),
```

Set `AnimationStyle.noAnimation` to disable animation.
```dart
    return MaterialApp(
      themeAnimationStyle: AnimationStyle.noAnimation,
```

## 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 material library.

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/134992
- https://github.com/flutter/flutter/pull/134993
- https://github.com/flutter/flutter/pull/134994

## Tests
 - Documentation only change.

Root predictive back (https://github.com/flutter/flutter/pull/120385) was reverted in https://github.com/flutter/flutter/pull/132167.  This PR is an attempt to reland it.

The reversion happened due to failed Google tests (b/295073110).

Breaking google testing 
revert of: https://github.com/flutter/flutter/pull/120385
b/295065534

This PR aims to support Android's predictive back gesture when popping the entire Flutter app.  Predictive route transitions between routes inside of a Flutter app will come later.

<img width="200" src="https://user-images.githubusercontent.com/389558/217918109-945febaa-9086-41cc-a476-1a189c7831d8.gif" />

### Trying it out

If you want to try this feature yourself, here are the necessary steps:

  1. Run Android 33 or above.
  1. Enable the feature flag for predictive back on the device under "Developer
     options".
  1. Create a Flutter project, or clone [my example project](https://github.com/justinmc/flutter_predictive_back_examples).
  1. Set `android:enableOnBackInvokedCallback="true"` in
     android/app/src/main/AndroidManifest.xml (already done in the example project).
  1. Check out this branch.
  1. Run the app. Perform a back gesture (swipe from the left side of the
     screen).

You should see the predictive back animation like in the animation above and be able to commit or cancel it.

### go_router support

go_router works with predictive back out of the box because it uses a Navigator internally that dispatches NavigationNotifications!

~~go_router can be supported by adding a listener to the router and updating SystemNavigator.setFrameworkHandlesBack.~~

Similar to with nested Navigators, nested go_routers is supported by using a PopScope widget.

<details>

<summary>Full example of nested go_routers</summary>

```dart
// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'package:go_router/go_router.dart';

import 'package:flutter/material.dart';
import 'package:flutter/scheduler.dart';

void main() => runApp(_MyApp());

class _MyApp extends StatelessWidget {
  final GoRouter router = GoRouter(
    routes: <RouteBase>[
      GoRoute(
        path: '/',
        builder: (BuildContext context, GoRouterState state) => _HomePage(),
      ),
      GoRoute(
        path: '/nested_navigators',
        builder: (BuildContext context, GoRouterState state) => _NestedGoRoutersPage(),
      ),
    ],
  );

  @override
  Widget build(BuildContext context) {
    return MaterialApp.router(
      routerConfig: router,
    );
  }
}

class _HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Nested Navigators Example'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text('Home Page'),
            const Text('A system back gesture here will exit the app.'),
            const SizedBox(height: 20.0),
            ListTile(
              title: const Text('Nested go_router route'),
              subtitle: const Text('This route has another go_router in addition to the one used with MaterialApp above.'),
              onTap: () {
                context.push('/nested_navigators');
              },
            ),
          ],
        ),
      ),
    );
  }
}

class _NestedGoRoutersPage extends StatefulWidget {
  @override
  State<_NestedGoRoutersPage> createState() => _NestedGoRoutersPageState();
}

class _NestedGoRoutersPageState extends State<_NestedGoRoutersPage> {
  late final GoRouter _router;
  final GlobalKey<NavigatorState> _nestedNavigatorKey = GlobalKey<NavigatorState>();

  // If the nested navigator has routes that can be popped, then we want to
  // block the root navigator from handling the pop so that the nested navigator
  // can handle it instead.
  bool get _popEnabled {
    // canPop will throw an error if called before build. Is this the best way
    // to avoid that?
    return _nestedNavigatorKey.currentState == null ? true : !_router.canPop();
  }

  void _onRouterChanged() {
    // Here the _router reports the location correctly, but canPop is still out
    // of date.  Hence the post frame callback.
    SchedulerBinding.instance.addPostFrameCallback((Duration duration) {
      setState(() {});
    });
  }

  @override
  void initState() {
    super.initState();

    final BuildContext rootContext = context;
    _router = GoRouter(
      navigatorKey: _nestedNavigatorKey,
      routes: [
        GoRoute(
          path: '/',
          builder: (BuildContext context, GoRouterState state) => _LinksPage(
            title: 'Nested once - home route',
            backgroundColor: Colors.indigo,
            onBack: () {
              rootContext.pop();
            },
            buttons: <Widget>[
              TextButton(
                onPressed: () {
                  context.push('/two');
                },
                child: const Text('Go to another route in this nested Navigator'),
              ),
            ],
          ),
        ),
        GoRoute(
          path: '/two',
          builder: (BuildContext context, GoRouterState state) => _LinksPage(
            backgroundColor: Colors.indigo.withBlue(255),
            title: 'Nested once - page two',
          ),
        ),
      ],
    );

    _router.addListener(_onRouterChanged);
  }

  @override
  void dispose() {
    _router.removeListener(_onRouterChanged);
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return PopScope(
      popEnabled: _popEnabled,
      onPopped: (bool success) {
        if (success) {
          return;
        }
        _router.pop();
      },
      child: Router<Object>.withConfig(
        restorationScopeId: 'router-2',
        config: _router,
      ),
    );
  }
}

class _LinksPage extends StatelessWidget {
  const _LinksPage ({
    required this.backgroundColor,
    this.buttons = const <Widget>[],
    this.onBack,
    required this.title,
  });

  final Color backgroundColor;
  final List<Widget> buttons;
  final VoidCallback? onBack;
  final String title;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: backgroundColor,
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(title),
            //const Text('A system back here will go back to Nested Navigators Page One'),
            ...buttons,
            TextButton(
              onPressed: onBack ?? () {
                context.pop();
              },
              child: const Text('Go back'),
            ),
          ],
        ),
      ),
    );
  }
}
```

</details>

### Resources

Fixes https://github.com/flutter/flutter/issues/109513
Depends on engine PR https://github.com/flutter/engine/pull/39208  
Design doc: https://docs.google.com/document/d/1BGCWy1_LRrXEB6qeqTAKlk-U2CZlKJ5xI97g45U7azk/edit#
Migration guide: https://github.com/flutter/website/pull/8952

Bump lower Dart SDK constraints to 3.0 & add class modifiers

Update ScrollableDetails for 2D scrolling

Separate theme logic

* Remove MediaQuery from WidgetsApp

* more

* fix test

* more deprecation notices.

* dart fix --apply

* manual fixes

* Use `curly_braces_in_flow_control_structures` for `material`

* include test/material

* add back removed comments

* Revert "Add default selection style (#100719)"

This reverts commit c8057bc5.

* update

* update

This reverts commit 5fd259be.