Fix CupertinoSwitch accessibility (#19927)

examples/flutter_gallery/lib/demo/cupertino/cupertino_switch_demo.dart

@@ -26,7 +26,9 @@ class _CupertinoSwitchDemoState extends State<CupertinoSwitchDemo> {
         child: new Column(
           mainAxisAlignment: MainAxisAlignment.spaceAround,
           children: <Widget>[
-            new Column(
+            new Semantics(
+              container: true,
+              child: new Column(
                 children: <Widget>[
                   new CupertinoSwitch(
                     value: _switchValue,
@@ -41,7 +43,10 @@ class _CupertinoSwitchDemoState extends State<CupertinoSwitchDemo> {
                   ),
                 ],
               ),
-            new Column(
+            ),
+            new Semantics(
+              container: true,
+              child: new Column(
                 children: const <Widget>[
                   const CupertinoSwitch(
                     value: true,
@@ -52,7 +57,10 @@ class _CupertinoSwitchDemoState extends State<CupertinoSwitchDemo> {
                   ),
                 ],
               ),
-            new Column(
+            ),
+            new Semantics(
+              container: true,
+              child: new Column(
                 children: const <Widget>[
                   const CupertinoSwitch(
                     value: false,
@@ -63,6 +71,7 @@ class _CupertinoSwitchDemoState extends State<CupertinoSwitchDemo> {
                   ),
                 ],
               ),
+            ),
           ],
         ),
       ),

packages/flutter/lib/src/cupertino/switch.dart

@@ -380,10 +380,11 @@ class _RenderCupertinoSwitch extends RenderConstrainedBox {
   void describeSemanticsConfiguration(SemanticsConfiguration config) {
     super.describeSemanticsConfiguration(config);
 
-    config.isSemanticBoundary = isInteractive;
     if (isInteractive)
       config.onTap = _handleTap;
-    config.isChecked = _value;
+
+    config.isEnabled = isInteractive;
+    config.isToggled = _value;
   }
 
   final CupertinoThumbPainter _thumbPainter = new CupertinoThumbPainter();