This reverts commit dc886418.

For some reason (probably a bad merge on my part), the return values for locale and locales are not the same on web as they are on other platforms for the PlatformDispatcher and SingletonFlutterWindow classes. The web versions are non-nullable and the others are nullable.

This converts them to be the same (non-nullable) values.

This re-lands #20496 and #21780 after fixing the semantics-enabling code that was causing the post-submit web_smoke_test to fail.

Below is the description from the original PR:

This is a PR for converting the dart:ui code in the engine to use a multi-window API. The goal here is to convert from the window singleton to an API that has the concept of multiple windows. Also, I'm matching up the new PlatformDispatcher class to talk directly to the PlatformConfiguration class in the engine. I'm not attempting to actually enable creating multiple windows here, just migrate to an API that has a concept of multiple windows. The multi-window API in this PR currently only ever creates one window.

The design doc for this change is here.

The major changes in this PR:

Move the platfom-specific attributes out of Window, and into the new PlatformDispatcher class that holds all of the platform state, so that the platform code need only update the configuration on this class.
Create FlutterView, FlutterWindow, and SingletonFlutterWindow classes to separate out the concepts of a view (of which there may be multiple in a window), a window (of which there may be multiple on a screen, and they host views), and a window where there is only ever expected to be one (this hosts the entire API of the former Window class, and will eventually be the type of the window singleton).
Next step after this PR lands:

Remove the Window class entirely (it is replaced by SingletonFlutterWindow). Some minor changes in the Framework are needed to switch to using SingletonFlutterWindow directly first.

The Window class still exists in this PR, but will be removed as soon as the framework is converted to point to the SingletonFlutterWindow class instead. They share the same API, just have different names (Window is currently a subclass of SingletonFlutterWindow). The intention is that the Window name will be freed up to use as a widget class name in the framework for managing windows. The singleton called window will remain, and keep the same API it has now.

* Revert "Fix documentation build for window changes. (#21780)"

This reverts commit 931a0468.

* Revert "Migration to PlatformDispatcher and multi-window (#20496)"

This reverts commit 85b0031f.

* Fix documentation build for window changes.

* Add missing interfaces for web_ui

This is a PR for converting the dart:ui code in the engine to use a multi-window API. The goal here is to convert from the window singleton to an API that has the concept of multiple windows. Also, I'm matching up the new PlatformDispatcher class to talk directly to the PlatformConfiguration class in the engine. I'm not attempting to actually enable creating multiple windows here, just migrate to an API that has a concept of multiple windows. The multi-window API in this PR currently only ever creates one window.

The design doc for this change is here.

The major changes in this PR:

Move the platfom-specific attributes out of Window, and into the new PlatformDispatcher class that holds all of the platform state, so that the platform code need only update the configuration on this class.
Create FlutterView, FlutterWindow, and SingletonFlutterWindow classes to separate out the concepts of a view (of which there may be multiple in a window), a window (of which there may be multiple on a screen, and they host views), and a window where there is only ever expected to be one (this hosts the entire API of the former Window class, and will eventually be the type of the window singleton).
Next step after this PR lands:

Remove the Window class entirely (it is replaced by SingletonFlutterWindow). Some minor changes in the Framework are needed to switch to using SingletonFlutterWindow directly first.

The Window class still exists in this PR, but will be removed as soon as the framework is converted to point to the SingletonFlutterWindow class instead. They share the same API, just have different names (Window is currently a subclass of SingletonFlutterWindow). The intention is that the Window name will be freed up to use as a widget class name in the framework for managing windows. The singleton called window will remain, and keep the same API it has now.

Reland #19396 with a fix for improper scale that was affecting internal tests

Tested: Ran all unittests, ran internal tests, and ran workstation on Fuchsia
BUG: 53062, 53063

This reverts commit 8d08e6c3.

This reverts commit 12a37478.

The fuchsia code around metrics and sizing was just sending this
information through a side-channel, when the engine already had the
information available. So, delete all of it to make future CLs simpler.

Additionally, the SceneUpdateContext has many unneccesary dependencies
re: metrics and PaintTasks. Break those to make future CLs simpler.

Tested: Ran all unittests and ran workstation on Fuchsia
BUG: 53062, 53063

* Manual merge or Dart into flutter a3815b6590...24c7666def

dart-lang/sdk@24c7666def Update tests that have the current version number hardcoded
dart-lang/sdk@f88ce7aef5 Increase Dart version to 2.10

* Update language version.

* Update license.

* Hpdate language version.

* Update license.

* use Object for operator==

* apply style guide for operator==

* apply null safety syntax to mobile dart:ui

1. Simple "GPU thread" to "raster thread" replacement.

2. Regex replace "GPU([\n\r\s]+//+ thread)" with "raster$1".

3. Regex replace "// gpu$" with "// raster".

4. Simple test change.

5. Run ci/format.sh

preserve Window callback zones

* opt out dart:ui from nnbd

* Annotate tests as well

Broke a fake implementation of Accessibility features in the framework.
While this change is unlikely to break any users in the wild, this is a
breaking change by the standards set out in our documentation and
should follow the breaking change announcement process. Details can be
found at https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes.

The error in question:

    Missing concrete implementation of 'getter AccessibilityFeatures.onOffSwitchLabels' • packages/flutter_test/test/window_test.dart:252:7 • non_abstract_class_inherits_abstract_member

This reverts commit d12f2a60.

This reverts commit 8ebb3184.

This doc update solves our TODO in https://github.com/flutter/flutter/pull/43676/files#diff-6a35a9a692c9af7473ba1a0a74c979c8R223