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

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.

* Analysis cleanup of the Dart in the Engine repo

* Update painting.dart

* Update painting.dart

To align with `SemanticsAction`.

* a11y cursor movement by character

* focus fixes

* doc fixes

* Identify text fields as such to a11y on Android

* Add focus flag

* Support for accessibility label and hint

* review comments

https://github.com/flutter/flutter/issues/11992

This action is triggered when the user swipes (in accessibility mode) to the last visible item of a scrollable list to bring that item fully on screen.

iOS implementation to follow.

* Adding SemanticsFlag.isSelected

* typo

This name is consistent with how the other `dart:` libraries are named now.

This patch prepares us to switch sending semantics information over mojom to
using dart:ui directly. Currently the recorded information is dropped on the
floor, but that will improve in future patches.