* Redesigns the interface between MouseTracker and RendererBinding&RenderView.
* Simplifies the structure of RenderMouseRegion.
* Extracts the common utility code between mouse_tracker_test and mouse_tracker_cursor_test.

After #31935, Some one sequence gesture recognizers requires at least one callback to be able to compete in the arena. This PR adds the a no-op callback in the gesture recognizer in the platform view when the gesture recognizer does not have any callbacks. This way, all the gesture recognizers in the platform view can compete in the arena.

Have AndroidViewController extend PlatformViewController and add support for hybrid platform views (#60320)

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* Uses a special cursor value MouseCursor.defer to mark deferring. MouseTrackerAnnotation.cursor is now non-null. The mouseCursor argument of widgets or render objects can be null, which indicates using the default value.
* Moves SystemMouseCursors.uncontrolled to MouseCursor.uncontrolled.
* Changes how MouseCursor.debugDescription is defined. Previously MouseCursor.toString returns $runtimeType($debugDescription), while now it returns $debugDescription. Implementations of classes are updated, except for the ones of MouseCursor.defer and MouseCursor.uncontrolled are simply "defer" and "uncontrolled".

Adds the basic framework for the mouse cursor system. 

* Adds MouseRegion.cursor
* Adds SystemMouseCursors
* Adds mouseCursor to some widgets

* Move mouse_tracking to rendering

* Move test

This adds support to PlatformViewLayer for handling hover events. Prior
to this, PlatformViewLayers only supported events forwarded by the
gesture recognizers associated with the PlatformViewRenderBox. Hover
events don't participate in gesture recognition and as such are dropped
in GestureBinding. That said, hover event processing in platform views
is expected for desktop and other platforms with hover event support.

This adds support for passing an optional MouseTrackerAnnotation to
PlatformViewLayer. PlatformViewRenderBox populates this with a mouse
tracker annotation that forwards hover events to
PlatformViewController.dispatchPointerEvent() for handling by users.

* implicit-casts:false in flutter/lib/src/rendering

* address review comments

* 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 appears to have been inadvertently added in #37497 (333c9618).

The code in question already exists in `_PlatformViewGestureMixin`, which
`PlatformViewRenderBox` (where it was added) mixes in.

Removes an unnecessary leading underscore from a parameter name. The
parameter type is library private, but parameters cannot be 'private'.

* painting and semantics

* more comments

* fixing ci

* review fixes

* add assert for id

* rename custom layer factory to layer builder

* review updates

* partial review fixes

* some doc updates

* more doc updates

* only expose getter for id in PlatformViewController

* doc updates/removing all the  references

* remove extra

* more doc updates

* some doc updates

* more doc fixes

* review fixes

Include the platformViewId of PlatformViews in the semantics tree. The accessibility bridge in the engine can use this id to steal the semantics nodes from the actual platform view and stick them into Flutter's semantics tree.

It is the iOS PlatformView counter part of https://github.com/flutter/flutter/pull/28953. It reverts the change in https://github.com/flutter/flutter/pull/28953/commits/5b5d6e89ec50f8f51766fb778d0e244f183a5000 and https://github.com/flutter/flutter/pull/28953/commits/03fd797eb8955561c2654b51035175202a25da9d.

https://github.com/flutter/flutter/issues/29302

* some space formattings

* always use blocks in if-else if a block is used

* format spaces in for and while

* allow multiline if conditions

* fix missing space

When a touch event that is in the bounds of a RenderUiKitView is absorbed by another render object,
the RenderUiKitView's handleEvent is not called for that object. On the platform side, the touch event hits the FlutterTouchInterceptingView which is waiting for a framework decision that never arrived on whether to reject or accept the gesture.

This change fixes the issue by having RenderUiKitView register a global PointerRoute, that is used to reject absorbed touch events.

* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

This was added to make sure we don't crash before the engine is rolled
with the system channel API addition and is no longer needed.

flutter/engine#7307 changes the engine side of embedded UIView to only
reject gestures when the framework sends a `rejectGesture` message, so
that gesture resolution can done after a touch sequence has ended (see
PR description for flutter/engine#7307 for more details).

This change makes the framework send a `rejectGesture` message to the
engine when a UiKitView rejects a gesture.

I'm planning to land this PR before the engine side change, so right now
it swallows the exception thrown if there is no engine implementation
for `rejectGesture` (which keeps us with the current behavior). After
this change lands I'll land the engine PR, and then clean up the part
that swallows the exception.

Adds a note mentioning that this is still a release preview.