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.

getTransformTo now includes ancestor in the transform it returns, except
for if ancestor is the root view, ensuring that the transform remains in
logical pixel space.

Rendering errors with root causes in the widget layer should have a reference to the widget (#32511)

* more ui-as-code

* address review comments

Default `semanticsEnabled` to true for `testWidgets` and fix associated bugs

Breaking change to extremely rarely used ParentDataWidget.debugDescribeInvalidAncestorChain api changing the return type of the method from String to DiagnosticsNode.

Adds the `textAlignVertical` param to TextField and InputDecorator, allowing arbitrary vertical positioning of text in its input.

This reverts commit 90500a5d.

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.

Added diagnostic output for needsCompositing to RenderObject diagnostics output.

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

* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

This reverts commit ea7d086e.

The revert did not fix the flakiness.

This reverts commit 5c1f1d76.

Reverting to see if that fixes flakiness of flutter_gallery__transition_perf_with_semantics in devicelab.

Fixes #20313.

This converts existing ## Sample code samples to {@tool sample}...{@end-tool} form.

Also:
1. Fixed a minor bug in analyze-sample-code.dart
2. Made the snippet tool only insert descriptions if the description is non-empty.
3. Moved the Card diagram to before the code sample.

For #21756

This re-lands 3c56e6c7 on top of a new Dart SDK that contains new mixin syntax fixes.