Fix FlutterPlatformViewSemanticsContainer retain cycle with SemanticsObject

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

@@ -576,10 +576,35 @@ - (void)dealloc {
   [super dealloc];
 }
 
-- (NSArray*)accessibilityElements {
-  return @[ _semanticsObject, _platformView ];
+#pragma mark - UIAccessibilityContainer overrides
+
+- (NSInteger)accessibilityElementCount {
+  // This container should only contain 2 elements:
+  // 1. The semantic object that represents this container.
+  // 2. The platform view object.
+  return 2;
 }
 
+- (nullable id)accessibilityElementAtIndex:(NSInteger)index {
+  FML_DCHECK(index < 2);
+  if (index == 0) {
+    return _semanticsObject;
+  } else {
+    return _platformView;
+  }
+}
+
+- (NSInteger)indexOfAccessibilityElement:(id)element {
+  FML_DCHECK(element == _semanticsObject || element == _platformView);
+  if (element == _semanticsObject) {
+    return 0;
+  } else {
+    return 1;
+  }
+}
+
+#pragma mark - UIAccessibilityElement overrides
+
 - (CGRect)accessibilityFrame {
   return _semanticsObject.accessibilityFrame;
 }

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

@@ -217,4 +217,16 @@ - (void)testShouldDispatchShowOnScreenActionForHidden {
   XCTAssertTrue(bridge->observations[0].action == flutter::SemanticsAction::kShowOnScreen);
 }
 
+- (void)testSemanticsObjectAndPlatformViewSemanticsContainerDontHaveRetainCycle {
+  fml::WeakPtrFactory<flutter::MockAccessibilityBridge> factory(
+      new flutter::MockAccessibilityBridge());
+  fml::WeakPtr<flutter::MockAccessibilityBridge> bridge = factory.GetWeakPtr();
+  SemanticsObject* object = [[SemanticsObject alloc] initWithBridge:bridge uid:1];
+  object.platformViewSemanticsContainer =
+      [[FlutterPlatformViewSemanticsContainer alloc] initWithSemanticsObject:object];
+  __weak SemanticsObject* weakObject = object;
+  object = nil;
+  XCTAssertNil(weakObject);
+}
+
 @end