update Tristate checkbox semantics to consider indeterminate as "unchecked" (#18297)

packages/flutter/lib/src/material/toggleable.dart

@@ -339,7 +339,7 @@ abstract class RenderToggleable extends RenderConstrainedBox {
     config.isEnabled = isInteractive;
     if (isInteractive)
       config.onTap = _handleTap;
-    config.isChecked = _value != false;
+    config.isChecked = _value == true;
   }
 
   @override

packages/flutter/test/material/checkbox_test.dart

@@ -202,6 +202,69 @@ void main() {
     expect(checkBoxValue, null);
   });
 
+  testWidgets('has semantics for tristate', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    await tester.pumpWidget(
+      new Material(
+        child: new Checkbox(
+          tristate: true,
+          value: null,
+          onChanged: (bool newValue) {},
+        ),
+      ),
+    );
+
+    expect(semantics.nodesWith(
+      flags: <SemanticsFlag>[
+        SemanticsFlag.hasCheckedState,
+        SemanticsFlag.hasEnabledState,
+        SemanticsFlag.isEnabled
+      ],
+      actions: <SemanticsAction>[SemanticsAction.tap],
+    ), hasLength(1));
+
+    await tester.pumpWidget(
+      new Material(
+        child: new Checkbox(
+          tristate: true,
+          value: true,
+          onChanged: (bool newValue) {},
+        ),
+      ),
+    );
+
+    expect(semantics.nodesWith(
+      flags: <SemanticsFlag>[
+        SemanticsFlag.hasCheckedState,
+        SemanticsFlag.hasEnabledState,
+        SemanticsFlag.isEnabled,
+        SemanticsFlag.isChecked,
+      ],
+      actions: <SemanticsAction>[SemanticsAction.tap],
+    ), hasLength(1));
+
+    await tester.pumpWidget(
+      new Material(
+        child: new Checkbox(
+          tristate: true,
+          value: false,
+          onChanged: (bool newValue) {},
+        ),
+      ),
+    );
+
+    expect(semantics.nodesWith(
+      flags: <SemanticsFlag>[
+        SemanticsFlag.hasCheckedState,
+        SemanticsFlag.hasEnabledState,
+        SemanticsFlag.isEnabled,
+      ],
+      actions: <SemanticsAction>[SemanticsAction.tap],
+    ), hasLength(1));
+
+    semantics.dispose();
+  });
+
   testWidgets('has semantic events', (WidgetTester tester) async {
     dynamic semanticEvent;
     bool checkboxValue = false;