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.

This reverts commit 120a1fc4.

Updated the golden file

## Description

Make `_RenderCustomClip`'s `clipBehavior` non-final so we can update it during `updateRenderObject`. This will support `clipBehavior` changes in hot reload.

## Related Issues

Fixes #30863

## Tests

I added the following tests:

* ClipRect updates clipBehavior in updateRenderObject
* ClipRRect updates clipBehavior in updateRenderObject
* ClipOval updates clipBehavior in updateRenderObject
* ClipPath updates clipBehavior in updateRenderObject
* PhysicalModel updates clipBehavior in updateRenderObject
* PhysicalShape updates clipBehavior in updateRenderObject

## Description

As we've introduced offset to the Opacity layer, we have to override
`applyTransform` to make Leader/FollowerLayer work correctly.

## Related Issues

Fixes https://github.com/flutter/flutter/issues/30587

Together with https://github.com/flutter/engine/pull/8585,
this test will also exercise test against
https://github.com/flutter/flutter/issues/30586.

## Tests

I added the following tests:
* text field selection toolbar renders correctly inside opacity

Fixes https://github.com/flutter/flutter/issues/31517

* Check for invalid elevation usage in the layer tree

This implements support for hovering mouse pointers, so that mice connected to Android devices, and ChromeOS devices running Android apps will work properly.

It teaches flutter_test about hover events, which required changing how they are created and used.

Also modifies AnnotatedRegion to allow a region that can be located someplace other than just the origin.

Along with tests for all of the above.

Fixes #5504

See https://github.com/flutter/flutter/issues/23890

Re-landing #23412 with a fix to the PlatformLayer's addToScene signature.

#23412 was broken by the change to Layer done in #23434.

It seemed green as the presubmits were done before #23434 was landed, and when #23412 landed it broke the build.

Reverts #23779