On Fuchsia, add a build flag for compositing OpacityLayers using the system
compositor vs Skia, which exposes a fastpath for opacity via Scenic.
This will only work under certain circumstances, in particular nested
OpacityLayers will not render correctly!

On Fuchsia, add a build flag for compositing PhysicalShapeLayers using
the system compositor vs Skia. Set to off by default, which restores
performant shadows on Fuchsia.

Remove the opacity exposed from ChildView, as that was added mistakenly.

Finally, we centralize the logic for switching between the
system-composited and in-process-composited paths inside of
ContainerLayer. We also centralize the logic for computing elevation
there. This allows the removal of many OS_FUCHSIA-specific code-paths.

Test: Ran workstation on Fuchsia; benchmarked before and after
Bug: 23711
Bug: 24163

* Fix broken tests

Add new optional named oldLayer arguments to all push* methods of the SceneBuilder class.

When not null oldLayer signals to the engine that the intent is to update a layer rendered in a previous frame. The engine may optionally use that signal to reuse the resources allocated for that layer in the previous frame. For example, on the Web we can reuse existing DOM nodes and some of their properties and move fewer nodes around the tree.

The return type of each push method has been tightened up. Instead of having all methods return the same EngineLayer type, each method has its own unique layer type, e.g. OffsetEngineLayer. oldLayer parameters match the returned type. This prevents the framework (and other developers using dart:ui directly) from accidentally supplying an engine layer of the wrong type.

SCN-947 #comment
SCN-1291 #comment

It should return void as SceneBuilder::addRetained is returning void.
Thanks yjbanov@ for finding this typo!

Dart lints added:
* Avoid optional new
* Avoid optional const
* Prefer single quotes
* Prefer default assignment `=`

Fixes flutter/flutter#31650

* Revert "fix toString (#8688)"

This reverts commit 9fa73367.

* Revert "Make Rect and RRect use 64 bit doubles, and make them const-able (#8565)"

This reverts commit c1231527.

* Make Rect and RRect 64bit and const-able

This reverts commit 0b36d3e2.

Fix the spelling errors in the dartdocs for the engine.

There are no functionality changes here, just documentation fixes.

The defaultClipBehavior is no longer used anywhere in the Flutter framework and Google3.

Most changes are trivial except pushTransform. In pushTransform, matrix4 is an Dart object and it has to be released before we can return a new Dart object (EngineLayer).

* Add missing entry-points.

* Fix lint

See https://github.com/flutter/flutter/pull/22566#discussion_r226082171
for why we add this.

This will be used for embedding UIViews on iOS.

Landing a no-op layer as a first incremental step to keep PRs small.

This reverts commit 74662ab6.

This should land after https://github.com/flutter/engine/pull/6442

* Add pragma vm:entry-point

Otherwise, an object may be both null and an instance of EnginieLayer at
the same time in Dart.

Reverts flutter/engine#6406

We need to fix the SkiaGPUObject issue of the raster cache SkImage before merging this PR.

To make the PR minimal, we currently only share the engine layer when `pushPhysicalShape` (for Fuchsia) or `pushOffset` (for `RepaintBoundary` and `Opacity`) are called. They should be sufficient for our short-term perf goal. In the future, we can incrementally share more engine layers with the framework.

https://github.com/flutter/flutter/issues/21756

For https://github.com/flutter/flutter/pull/21619#discussion_r220390088

The `dart_vm_entry_points.txt` file is now deprecated in favor of annotations.
Support for parsing and generating the entry-points files will be removed from Dart shortly.

This is a follow up of https://github.com/flutter/engine/pull/5853

This is needed to avoid jank when resizing an embedded Android view.
See
https://github.com/flutter/flutter/issues/19572#issuecomment-410400724

The is a follow up of https://github.com/flutter/engine/pull/5853

For flutter/flutter#18057

So our future pull requests won't be polluted by the white space changes.

* pass shadow_color from dart:ui to physical layer

* default shadow color to SK_ColorBLACK

* add doc comments to pushPhysicalShape

* Make native wrapper classes non-abstract as they are instantiated from native code
* Make constructors of native wrapper classes private

* Analysis cleanup of the Dart in the Engine repo

* Update painting.dart

* Update painting.dart

This reverts commit d8740c70.