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

This relands https://github.com/flutter/engine/pull/4629 with a
tonic roll to fix the build.

* implement iOS text field editing in a11y mode

* address Chinmay's comments

* replace node in child list when changing type

* Fixes to make https://dart-review.googlesource.com/c/sdk/+/36641 work with Flutter

* Roll dart

* Update license

* Roll dart

* Update license

* Add libraries.json to dependencies

This reverts commit 8f638591.

This adds support for semantics traversal ordering.

It is a companion to flutter/flutter#14060, adding support for a sortIndex in the semantics data passed to the engine.

Addresses flutter/flutter#12187

See https://github.com/flutter/flutter/pull/14275 for framework side change.

Also includes some minor clean-ups for consistency. 

Required for https://github.com/flutter/flutter/issues/13469.

Fixes https://github.com/flutter/flutter/issues/14082

* Analysis cleanup of the Dart in the Engine repo

* Update painting.dart

* Update painting.dart

Fixes https://github.com/flutter/flutter/issues/7201.

The change from `new Rect.largest()` to `Rect.largest` saves a few
allocations since we'll be using these with every Canvas.

Since we've got a vector class, and it has a getter for the magnitude,
why not also a getter for the angle.

To align with `SemanticsAction`.

This reverts commit d8740c70.

Add support for --strong option in the engine, create a strong mode version of the platform file (#4504)

* Add a --strong option to the front end server so we can use strong mode with preview-dart-2.

* Plumb the --strong option through the dart controller into the VM.

* - Build a strong version of platform.dill for use with the engine.
- Fix a strong mode static error in the assert statement

* Enable asserts when running debug version even in strong mode.

* Use the correct platform dill file for linking when doing the aot builds.

* Fix formatting issue.

* Adjust PhysicalModelLayer to use an abstract shape, and provide concrete RRect and Path shape implementations

* add a pushPhysicalShape to scene_builder and compositing.dart

Fixes https://github.com/flutter/flutter/issues/12377.

Reverts the revert in #4448 with fixes to pass on the bot.

This change will require framework changes in flutter/flutter#13490.

This reverts commit 59c3a37e.