* Semantic hit test order for Slivers

* ++

* fix double space

* refactoring

* fix test

* comment fix

* tests

* remove accidental debugDumpSemanticsTree

* review comments

* fix button

* 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

* Exclude modal barrier from semantics on Android

* Add tests

Adds a widgets that blocks all semantics of widgets below it in paint order within the same container (#10425)

Previously we used logical pixels. This made the accessibility metrics
tiny on modern devices, since the OS works in physical units.

Also add a bit more debugging info and some docs.

Also:

 * Add three explicit sizing modes to Stack for non-positioned
   children: loose, expand, and passthrough. (All three are used.)

 * Fix a bug whereby layers would try to paint in the same frame as
   they were removed from layout (but not detached).

 * Fix a bug whereby Offstage wasn't properly marking the parent dirty
   when changing its sizedByParent flag.

 * Explicitly make Overlay expand non-positioned children.

 * Explicitly have InputDecoration pass through the constraints from
   its Row to its Stack children.

Fixes #7734

This match switches the framework to use the semantics backend in `dart:ui`
rather than the Mojo backend.