[iOS][A11Y] fix hittest with non-SemanticsObject (flutter#44014)

PlatformViewSemanticsContainer did not implement the nativeAccessibility method, leads to a nil object being returned. 

Fixes flutter/flutter#131014

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

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

@@ -547,17 +547,16 @@ - (bool)containsPoint:(CGPoint)point {
 }
 
 // Finds the first eligiable semantics object in hit test order.
-- (SemanticsObject*)search:(CGPoint)point {
+- (id)search:(CGPoint)point {
   // Search children in hit test order.
   for (SemanticsObject* child in [self childrenInHitTestOrder]) {
     if ([child containsPoint:point]) {
-      SemanticsObject* childSearchResult = [child search:point];
+      id childSearchResult = [child search:point];
       if (childSearchResult != nil) {
         return childSearchResult;
       }
     }
   }
-
   // Check if the current semantic object should be returned.
   if ([self containsPoint:point] && [self isFocusable]) {
     return self.nativeAccessibility;
@@ -879,6 +878,10 @@ - (void)dealloc {
   [super dealloc];
 }
 
+- (id)nativeAccessibility {
+  return _platformView;
+}
+
 #pragma mark - UIAccessibilityContainer overrides
 
 - (NSArray*)accessibilityElements {

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

@@ -203,6 +203,54 @@ - (void)testAccessibilityHitTestNoFocusableItem {
   XCTAssertNil(hitTestResult);
 }
 
+- (void)testAccessibilityHitTestSearchCanReturnPlatformView {
+  fml::WeakPtrFactory<flutter::AccessibilityBridgeIos> factory(
+      new flutter::MockAccessibilityBridge());
+  fml::WeakPtr<flutter::AccessibilityBridgeIos> bridge = factory.GetWeakPtr();
+  SemanticsObject* object0 = [[SemanticsObject alloc] initWithBridge:bridge uid:0];
+  SemanticsObject* object1 = [[SemanticsObject alloc] initWithBridge:bridge uid:1];
+  SemanticsObject* object3 = [[SemanticsObject alloc] initWithBridge:bridge uid:3];
+  UIView* platformView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];
+  FlutterPlatformViewSemanticsContainer* platformViewSemanticsContainer =
+      [[FlutterPlatformViewSemanticsContainer alloc] initWithBridge:bridge
+                                                                uid:1
+                                                       platformView:platformView];
+
+  object0.children = @[ object1 ];
+  object0.childrenInHitTestOrder = @[ object1 ];
+  object1.children = @[ platformViewSemanticsContainer, object3 ];
+  object1.childrenInHitTestOrder = @[ platformViewSemanticsContainer, object3 ];
+
+  flutter::SemanticsNode node0;
+  node0.id = 0;
+  node0.rect = SkRect::MakeXYWH(0, 0, 200, 200);
+  node0.label = "0";
+  [object0 setSemanticsNode:&node0];
+
+  flutter::SemanticsNode node1;
+  node1.id = 1;
+  node1.rect = SkRect::MakeXYWH(0, 0, 200, 200);
+  node1.label = "1";
+  [object1 setSemanticsNode:&node1];
+
+  flutter::SemanticsNode node2;
+  node2.id = 2;
+  node2.rect = SkRect::MakeXYWH(0, 0, 100, 100);
+  node2.label = "2";
+  [platformViewSemanticsContainer setSemanticsNode:&node2];
+
+  flutter::SemanticsNode node3;
+  node3.id = 3;
+  node3.rect = SkRect::MakeXYWH(0, 0, 200, 200);
+  node3.label = "3";
+  [object3 setSemanticsNode:&node3];
+
+  CGPoint point = CGPointMake(10, 10);
+  id hitTestResult = [object0 _accessibilityHitTest:point withEvent:nil];
+
+  XCTAssertEqual(hitTestResult, platformView);
+}
+
 - (void)testAccessibilityScrollToVisible {
   fml::WeakPtrFactory<flutter::MockAccessibilityBridge> factory(
       new flutter::MockAccessibilityBridge());