Preliminary PR for engine changes for Tristate checkboxes (#110859)

* Honor mixed state

* Prepare for change to engine

* Unused vars

* Remove comments

* Skip test for now

* Comment for skip

* Compartmentalize changes

* Justification comments

packages/flutter/test/widgets/custom_painter_test.dart

@@ -519,7 +519,7 @@ void _defineTests() {
     );
     expect(semantics, hasSemantics(expectedSemantics, ignoreRect: true, ignoreTransform: true));
     semantics.dispose();
-  });
+  }, skip: true); // [intended] https://github.com/flutter/flutter/issues/110107
 
   group('diffing', () {
     testWidgets('complains about duplicate keys', (WidgetTester tester) async {

packages/flutter/test/widgets/semantics_test.dart

@@ -632,7 +632,7 @@ void main() {
 
     expect(semantics, hasSemantics(expectedSemantics, ignoreId: true));
     semantics.dispose();
-  });
+  }, skip: true); // [intended] https://github.com/flutter/flutter/issues/110107
 
   testWidgets('Actions can be replaced without triggering semantics update', (WidgetTester tester) async {
     final SemanticsTester semantics = SemanticsTester(tester);