* 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

Use mockito in the Dart 2 compliant way:

* use typed(...) in appropriate places to wrap matchers like any;
* don't pass matcher directly into the calls - use `argThat(matcher)`.

Also rename paintColorMatcher to hasColor to match how other similar
functions are named and how the code reads.

Using a void list literal in this case is valid and correct, and will be
required for dart 2.

Note: void can be inferred here, but followed the lints to write it
explicitly even though that's arguably a bit weird in this case and less
backwards compatible.