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

Addresses flutter/flutter#18028 for the engine repo, and synchronizes the analysis_options.yaml files between the engine and the flutter/flutter repo.

* split `children` into two ordered lists: traversal and hit test

* address comments

* reduce node object byte size

* link to DebugSemanticsDumpOrder

AccessibilityBridge support for edge triggered semantics (iOS + Android) 

It seems that setTraversalBefore doesn't work as well as setTraversalAfter for some reason, although I'm using them the same way. Some apps would lock up TalkBack when traversing if setTraversalBefore was set, but not with the equivalent setTraversalAfter.

It's not entirely clear why this is, but I'm going with this to at least get it fixed for apps we know about.

Addresses flutter/flutter#14600

See also flutter/flutter#14607

* 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