Revert "Add CompositedTransformFollower.{followerAnchor, leaderAnchor} for custom anchoring (#64930)" (#65871)

This reverts commit 2fbb529d.

SceneBuilder.addPicture() is implemented in C++, and if `picture`
is null, it will seg-fault. This catches the problem before that
happens.

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* Move mouse_tracking to rendering

* Move test

This reverts commit c235998a.

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

Fixes https://github.com/flutter/flutter/issues/45587

No code changes, just comments: I spell-checked all the comments in the repo.

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

* Adds a new parameter bool opaque to MouseRegion
* The same to RenderMouseRegion and AnnotatedRegionLayer
* Add findAnnotations to Layer 

I noticed that we were pretty inconsistent with the way that we checked the value of clipBehavior in the framework, so I normalized the usages and updated docs where necessary.

This is a breaking change if you used to pass null explicitly to FlatButton, OutlineButton or RaisedButton constructors, expecting to get Clip.none. It will now assert if you do that. Existing implementations that pass null implicitly by not specifying clipBehavior won't need to change their call sites. It always implicitly defaulted to Clip.none before, and it will continue to do that, it's only places where it was explicitly set to null in order to get the implicit default that it will fail.

Teach Layer and its implementations, RenderObject and its implementations, and PaintingContext to reuse engine layers. The idea is that a concrete RenderObject creates a Layer and holds on to it as long as it needs it (i.e. when it is composited, and the layer type does not change). In return, each Layer object holds on to an EngineLayer and reports it to the engine via addRetained and oldLayer. This allows the Web engine to reuse DOM elements across frames. Without it, each frame drops all previously rendered HTML and regenerates it from scratch.

(and fix grammar)

See: https://github.com/flutter/flutter/pull/21826