* Add semantics sort key for modal scope and modal barrier

* fix test

* fix test

* fix test

* fix space

* fix more tests

* addressing comments

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

* 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

* Future<void> main

* Future<void>.delayed

* prefer_void_to_Null

* address review comments

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Set text direction in semantics along with Directionality

* update tests for the extra SemanticsNode

New a11y traversal:

- sort direct sibling SemanticsNodes only
- use new sorting algorithm
- implement RTL
- test semantics in traversal order by default
- add AppBar traversal test
- breaking: remove nextNodeId/previousNodeId from the framework
- breaking: remove DebugSemanticsDumpOrder.geometricOrder

Semantics object support for edge triggered semantics 

* a11y: remove SemanticsSortOrder; sort locally only; semanticsOwner post-test check

* update accessibility test framework

- default nextNodeId/previousNodeId to -1
- stop treating null as opt-out from value testing
- add `id`, `TestSemantics.root`, and `tags` to the suggested code in the TestSemantics failure message
- fix a small bug with raw string escaping
- update all tests accordingly

* fix sortKey doc

* prefer const over final

Re-apply "Send nextId in addition to previousId for traversal order (#14676)"

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.

Fixes #14600

This adds an API for defining the semantic node traversal order.

It adds a sortOrder argument to the Semantics widget, which is a class that can define a list of sort keys to sort on. The keys are sorted globally so that an order that doesn't have to do with the current widget hierarchy may be defined.

It also adds a shortcut sortKey argument to the Semantics widget that simply sets the sortOrder to just contain that key.

The platform side (flutter/engine#4540) gets an additional member in the SemanticsData object that is an integer describing where in the overall order each semantics node belongs. There is an associated engine-side change that takes this integer and uses it to order widgets for the platform's accessibility services.

to align with `SemanticsAction` and other enum-like types.

* make date picker accessible

* make test file lookup location-independent

* address some comments

* always wrap in IgnorePointer

* no bitmasks for flags and actions

* recommend List<*>