* Revert "Revert "MouseRegion enter/exit event can be triggered with button pressed (#81148)" (#81557)"

* enable null_check_on_nullable_type_parameter and tighten_type_of_initializing_formals

* fix CI

* 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.

* Add punctuation for unterminated sentences in doc comments

* Addressing review nit

* 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 converts Diagnosticable to be a mixin instead of an abstract class, so that it can be used to add diagnostics to classes which already have a base class.

It leaves in place the DiagnosticableMixin mixin, since there are some plugins that are still using it, and removing it would mean that those plugins wouldn't work with master branch Flutter anymore. DiagnosticableMixin will be removed once this mixin version of Diagnosticable makes its way to the stable branch.

This reverts commit c235998a.

* Revert test pointer

* Synthesize new state

This PR rewrites MouseTracker's lifecycle, so that mouse callbacks are all triggered in post frame, instead of the current one where some are triggered during the build phase. This PR also changes the onExit callback to MouseRegion, RenderMouseRegion, and MouseTrackerAnnotation, so that it is no longer triggered on dispose.

* 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

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

* address review comments

* Replace hard break of Layer.find/All with findAllAnnotations
* Deprecate findAll

* Revert "Revert "Rewrite MouseTracker's tracking and notifying algorithm (#42031)" (#42478)"

This reverts commit eede7929.

* Fix tests

* Redesigns the algorithm with which MouseTracker keeps track of states and dispatches events. 

* Make sendMouseNotifications private

This fixes the mouse hover code to not schedule frames with every mouse move.

Before this, it would schedule a post frame callback, and then schedule a frame immediately, even if there was nothing that needed to be updated. Now it will schedule checks for mouse position updates synchronously, unless there's a new annotation, and skip scheduling a new frame in all cases. It has to be async in the case of a new annotation (i.e. a new MouseRegion is added), since when the annotation is added, it hasn't yet painted, and it can't hit test against the new layer until after the paint, so in that case it schedules a post frame callback, but since it's already building a frame when it does that, it doesn't need to schedule a frame.

The code also used to do mouse position checks for all mice if only one mouse changed position. I fixed this part too, so that it will only check position for the mouse that changed.

This PR solves two problems: currently, the onExit is called for a mouse pointer the moment the removal message is received, except that by the time it actually calls it, there is no _lastEvent for it in the mouse tracker (it's already been removed), resulting in an event being passed to the onExit that contains nulls for the position. Also, removePointer events don't actually get created with a position, although they easily could be, so that even the the _lastEvent in the mouse tracker were still populated, it would still give a null position and delta.

This PR adds support for the position and delta in a PointerRemovedEvent, and populates them. In addition, when a remove event is received, it doesn't actually remove the pointer until the mouse position check that gets scheduled actually happens.