This is follow-up to https://github.com/flutter/flutter/pull/15537 to make dart1 coverage test happy.

* 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.

* Identify CHeckBox and RadioButton as such

* review feedback

`RenderSemanticsGestureHandler` is no longer a semantics boundary, which allows us to correctly mark disabled buttons as disabled without having their semantics size and semantics node id change unexpectedly.

Fixes https://github.com/flutter/flutter/issues/12589.
Fixes https://github.com/flutter/flutter/issues/11991.
See also https://github.com/flutter/flutter/issues/11993.

This change also required some refactoring to how we deal with `twoPaneSemantics` scrolling as it previously relied on `RenderSemanticsGestureHandler` being a semantics boundary. This should also make the underlying logic easier to understand.

In addition, the following minor changes are included in this PR:
* Removal of orphaned and unused `SemanticsConfiguration.isMergingDescendantsIntoOneNode`.
* Logic optimizations for `markNeedsSemanticsUpdate` .
* Fix for edge case where `MergeSemantics` failed to merge semantics.
* Use of emojis to better indicate leaf merging in the printed semantics tree.
* Better assert message for adding invisible child semantics nodes.
* Make some semantics tests robuster by not relying on creation order of SemanticsNode ids across test boundaries.

Fixes https://github.com/flutter/flutter/issues/13943.

Fixes https://github.com/flutter/flutter/issues/11993

**This PR contains a breaking API change:**
`SemanticsConfiguration.addAction()` has been removed and replaces by action-specific setters (`onTap`, `onLongPress`, etc.) to take care of the fact that some action handlers (those, who take arguments) have different signatures.

* ensures that only semantics boundaries will be added to owner._nodesNeedingSemantics as expected by compiler.
* no longer throws assert if markNeedsSemanticsUpdate is called on non-semantic-boundary render object with a non-semantic-boundary parent.
* Fixes #13109.
* removes onlyLocalUpdates from markNeedsSemanticsUpdate as its no longer needed.

* Identify text fields as such to a11y

* focus

* make travis happy

* review comments

* Support value announcment for increase/decrease

* ++

* review comments about docs

* Support accessibility labels and hints

* more tests

* ++

* review comments

* fix merge

* test fix

* Without checking compatibility

* ++

* ++

* ++

* more docs

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

* Oct 12 10:12am

* implicit_semantics_test.dart passes

* refactoring

* works in nice

* minor rename

* more doc comments

* to be explicit check better

* fix test

* ++

* ++

* semantics_9_test (BlockSemantics) and implicit_semantics_test are passing

* doc updates

* tiny refactor

* fix static errors in tests

* fix gesture detector

* ++

* ++

* geometry

* ++

* remove noGeometry

* revert test

* +

* all tests but scrolling/clipping pass

* clipping works

* scrolling halfway

* sliver tests pass

* ALL TESTS PASS

* SemanticsNode changed

* docs and tiny fixes

* card test

* more doc comments

* remove missed print

* more tests

* make test pass on Linux

* remove changes to intellij proj file

* review comments

* Drop invisible SemanticsNodes from tree

A node is invisible if it is outside of the bounds of the screen and if it is not merged into its (partially) visible parent.

Also in this PR: only set `wasAffectedByClip` to true if the nodes has actually been clipped.

* Fix other failing tests

* renaming

* review feedback

* more doc

* ++

* Allow unmerging of SemanticsNodes

* test passing

* ++

* remove prints

* doc comments

* rectify comment

* review comments

Diagnostics object is.

* Refactor SemanticsData and SemanticsNode to use Diagnosticable.

* Switch toStringDeep to take named parameters.

* Semantic hit test order for Slivers

* ++

* fix double space

* refactoring

* fix test

* comment fix

* tests

* remove accidental debugDumpSemanticsTree

* review comments

* fix button

* refactor to ensureAction + some related doc fixes

* Update docs for markNeedsSemanticsUpdate

* rewording

* rewording

* ensureAction test

* ensureAction test

* ensureAction test

* more tests

* refactor to allways reset node

* tiny fixes

* more test

* doc fixes

* one more test

* review comments

* refactor to assembleSemanticNode

* ++

* cleanup

* fix test

* add note

* review comments

* review feedback

* import fix

* another import fix

* refactor to ensure tag

* tests, tests, tests

* analyzer fixes

* review comments