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

Updates documentation and non-public API to use American spellings for
consistency with the rest of the codebase.

No changes to behaviour... other than how it's spelt.

This relands flutter/flutter#31701 with missing const added

This reverts commit 549b4126.

## Description

These will help identify where the matrix starts to get wrong. 

Also fixed `RenderFittexBox` to no longer paint with empty child which previously triggered invalid matrix computations (NaN with dividing by 0). See also https://github.com/flutter/flutter/pull/7489

## Related Issues

https://github.com/flutter/flutter/issues/31650
https://github.com/flutter/flutter/issues/31700
https://github.com/flutter/flutter/issues/7431

## Tests

* RenderFittedBox does not paint with empty sizes

This fixes a benchmark regression introduced in #32350. The performance is improved by just reverting the Layer.find<S> routines to use their old definitions, instead of defining them in terms of the findAll<S> lazy iterators.

Fixes #32387

This changes Listener to trigger enter/move/exit in all Listeners below the pointer, not just the leaf region (the first region hit). This is because we need to allow listeners to be nested so that, say, a widget that handles changing color on hover, but also is wrapped in a Tooltip (that handles hover) can trigger both actions, not just one.

To that end, I added a findAll to Layer, similar to the existing find method that was previously used. It returns an iterator over annotated layers which match the given data type.

Since the findAll is implemented as returning an Iterable (and is sync*), I re-implemented the find routines as just returning the first result from findAll, since that should be just as efficient, and would then prevent duplication in the implementation.

Make FlutterError objects more structured so they can be displayed better in debugging tools such as Dart DevTools.