Refactor Semantics in preparation for ARC migration (flutter#52729)

Split the too-large `Semantics` [MRC to ARC migration](flutter#137801) into two PRs: this one, which refactors, and the next which will actually do the migration.

1. Use properties instead of their backing ivars (except in the usual init, dealloc, getters/setters)
2. For Foundation collections prefer `copy` over `strong`.
3. Dot notation for properties
4. Change `privateSetParent:` to instead use a `readwrite` property in `SemanticsObject ()`. 
5. Switch the `semanticsObject` property from `weak` to `retain` to get the synthesized property (keeping it as `weak` is a compilation error in MRC) but I'll swap it back to a `weak` in the ARC migration PR coming next.
6. `SemanticsObjectTest` fails on my machine and passes on CI.  Switched the cleaner `CGRectEqualToRect` (and related) checks to instead assert x, y, width, height so we can see the value when it fails:
```
((CGSizeEqualToSize( scrollView.contentSize, CGSizeMake((w + scrollExtentMax) * effectivelyScale, h * effectivelyScale))) is true) failed
```
becomes:
```
((scrollView.contentSize.height) equal to (h * effectivelyScale)) failed: ("33.3333333333") is not equal to ("33.333336")
```
Use `XCTAssertEqualWithAccuracy` now that I can see it's a floating point precision issue.

Author: jmagman

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

@@ -25,81 +25,81 @@ - (UIView*)hitTest:(CGPoint)point withEvent:(UIEvent*)event {
 // UIScrollView class, the base class.
 
 - (BOOL)isAccessibilityElement {
-  if (![_semanticsObject isAccessibilityBridgeAlive]) {
+  if (![self.semanticsObject isAccessibilityBridgeAlive]) {
     return NO;
   }
 
-  if ([_semanticsObject isAccessibilityElement]) {
+  if (self.semanticsObject.isAccessibilityElement) {
     return YES;
   }
   if (self.contentSize.width > self.frame.size.width ||
       self.contentSize.height > self.frame.size.height) {
     // In SwitchControl or VoiceControl, the isAccessibilityElement must return YES
     // in order to use scroll actions.
-    return ![_semanticsObject bridge]->isVoiceOverRunning();
+    return ![self.semanticsObject bridge]->isVoiceOverRunning();
   } else {
     return NO;
   }
 }
 
 - (NSString*)accessibilityLabel {
-  return [_semanticsObject accessibilityLabel];
+  return self.semanticsObject.accessibilityLabel;
 }
 
 - (NSAttributedString*)accessibilityAttributedLabel {
-  return [_semanticsObject accessibilityAttributedLabel];
+  return self.semanticsObject.accessibilityAttributedLabel;
 }
 
 - (NSString*)accessibilityValue {
-  return [_semanticsObject accessibilityValue];
+  return self.semanticsObject.accessibilityValue;
 }
 
 - (NSAttributedString*)accessibilityAttributedValue {
-  return [_semanticsObject accessibilityAttributedValue];
+  return self.semanticsObject.accessibilityAttributedValue;
 }
 
 - (NSString*)accessibilityHint {
-  return [_semanticsObject accessibilityHint];
+  return self.semanticsObject.accessibilityHint;
 }
 
 - (NSAttributedString*)accessibilityAttributedHint {
-  return [_semanticsObject accessibilityAttributedHint];
+  return self.semanticsObject.accessibilityAttributedHint;
 }
 
 - (BOOL)accessibilityActivate {
-  return [_semanticsObject accessibilityActivate];
+  return [self.semanticsObject accessibilityActivate];
 }
 
 - (void)accessibilityIncrement {
-  [_semanticsObject accessibilityIncrement];
+  [self.semanticsObject accessibilityIncrement];
 }
 
 - (void)accessibilityDecrement {
-  [_semanticsObject accessibilityDecrement];
+  [self.semanticsObject accessibilityDecrement];
 }
 
 - (BOOL)accessibilityScroll:(UIAccessibilityScrollDirection)direction {
-  return [_semanticsObject accessibilityScroll:direction];
+  return [self.semanticsObject accessibilityScroll:direction];
 }
 
 - (BOOL)accessibilityPerformEscape {
-  return [_semanticsObject accessibilityPerformEscape];
+  return [self.semanticsObject accessibilityPerformEscape];
 }
 
 - (void)accessibilityElementDidBecomeFocused {
-  [_semanticsObject accessibilityElementDidBecomeFocused];
+  [self.semanticsObject accessibilityElementDidBecomeFocused];
 }
 
 - (void)accessibilityElementDidLoseFocus {
-  [_semanticsObject accessibilityElementDidLoseFocus];
+  [self.semanticsObject accessibilityElementDidLoseFocus];
 }
 
 - (id)accessibilityContainer {
-  return [_semanticsObject accessibilityContainer];
+  return self.semanticsObject.accessibilityContainer;
 }
 
 - (NSInteger)accessibilityElementCount {
-  return [[_semanticsObject children] count];
+  return self.semanticsObject.children.count;
 }
 
 @end

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

@@ -38,7 +38,7 @@ constexpr float kScrollExtentMaxForInf = 1000;
  * The parent of this node in the node tree. Will be nil for the root node and
  * during transient state changes.
  */
-@property(nonatomic, assign) SemanticsObject* parent;
+@property(nonatomic, assign, readonly) SemanticsObject* parent;
 
 /**
  * The accessibility bridge that this semantics object is attached to. This
@@ -64,13 +64,13 @@ constexpr float kScrollExtentMaxForInf = 1000;
  * Direct children of this semantics object. Each child's `parent` property must
  * be equal to this object.
  */
-@property(nonatomic, strong) NSArray<SemanticsObject*>* children;
+@property(nonatomic, copy) NSArray<SemanticsObject*>* children;
 
 /**
  * Direct children of this semantics object in hit test order. Each child's `parent` property
  * must be equal to this object.
  */
-@property(nonatomic, strong) NSArray<SemanticsObject*>* childrenInHitTestOrder;
+@property(nonatomic, copy) NSArray<SemanticsObject*>* childrenInHitTestOrder;
 
 /**
  * The UIAccessibility that represents this object.
@@ -231,7 +231,7 @@ constexpr float kScrollExtentMaxForInf = 1000;
                                  bridge:(fml::WeakPtr<flutter::AccessibilityBridgeIos>)bridge
     NS_DESIGNATED_INITIALIZER;
 
-@property(nonatomic, weak) SemanticsObject* semanticsObject;
+@property(nonatomic, assign) SemanticsObject* semanticsObject;
 
 @end