Add support for exposing accessibility identifier as accessibilityIdentifier on iOS (#48858)

This PR is a sibling of #47961 but for iOS

Author: bartekpacia

shell/platform/darwin/ios/framework/Source/SemanticsObject.mm

@@ -524,6 +524,17 @@ - (BOOL)onCustomAccessibilityAction:(FlutterCustomAccessibilityAction*)action {
   return YES;
 }
 
+- (NSString*)accessibilityIdentifier {
+  if (![self isAccessibilityBridgeAlive]) {
+    return nil;
+  }
+
+  if ([self node].identifier.empty()) {
+    return nil;
+  }
+  return @([self node].identifier.data());
+}
+
 - (NSString*)accessibilityLabel {
   if (![self isAccessibilityBridgeAlive]) {
     return nil;

shell/platform/darwin/ios/framework/Source/SemanticsObjectTest.mm

@@ -508,6 +508,20 @@ - (void)testFlutterScrollableSemanticsObjectIsHiddenWhenVoiceOverIsRunning {
   XCTAssertFalse(scrollView.isAccessibilityElement);
 }
 
+- (void)testFlutterSemanticsObjectHasIdentifier {
+  flutter::testing::MockAccessibilityBridge* mock = new flutter::testing::MockAccessibilityBridge();
+  mock->isVoiceOverRunningValue = true;
+  fml::WeakPtrFactory<flutter::AccessibilityBridgeIos> factory(mock);
+  fml::WeakPtr<flutter::AccessibilityBridgeIos> bridge = factory.GetWeakPtr();
+
+  flutter::SemanticsNode node;
+  node.identifier = "identifier";
+
+  FlutterSemanticsObject* object = [[FlutterSemanticsObject alloc] initWithBridge:bridge uid:0];
+  [object setSemanticsNode:&node];
+  XCTAssertTrue([object.accessibilityIdentifier isEqualToString:@"identifier"]);
+}
+
 - (void)testFlutterScrollableSemanticsObjectWithLabelValueHintIsNotHiddenWhenVoiceOverIsRunning {
   flutter::testing::MockAccessibilityBridge* mock = new flutter::testing::MockAccessibilityBridge();
   mock->isVoiceOverRunningValue = true;