Avoid using Dart_New for semantics

lib/ui/semantics.dart

@@ -805,7 +805,12 @@ class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
   ///
   /// The returned object can be passed to [Window.updateSemantics] to actually
   /// update the semantics retained by the system.
-  SemanticsUpdate build() native 'SemanticsUpdateBuilder_build';
+  SemanticsUpdate build() {
+    final SemanticsUpdate semanticsUpdate = SemanticsUpdate._();
+    _build(semanticsUpdate);
+    return semanticsUpdate;
+  }
+  void _build(SemanticsUpdate outSemanticsUpdate) native 'SemanticsUpdateBuilder_build';
 }
 
 /// An opaque object representing a batch of semantics updates.

lib/ui/semantics/semantics_update.cc

@@ -20,11 +20,12 @@ IMPLEMENT_WRAPPERTYPEINFO(ui, SemanticsUpdate);
 
 DART_BIND_ALL(SemanticsUpdate, FOR_EACH_BINDING)
 
-fml::RefPtr<SemanticsUpdate> SemanticsUpdate::create(
-    SemanticsNodeUpdates nodes,
-    CustomAccessibilityActionUpdates actions) {
-  return fml::MakeRefCounted<SemanticsUpdate>(std::move(nodes),
-                                              std::move(actions));
+void SemanticsUpdate::create(Dart_Handle semantics_update_handle,
+                             SemanticsNodeUpdates nodes,
+                             CustomAccessibilityActionUpdates actions) {
+  auto semantics_update = fml::MakeRefCounted<SemanticsUpdate>(
+      std::move(nodes), std::move(actions));
+  semantics_update->AssociateWithDartWrapper(semantics_update_handle);
 }
 
 SemanticsUpdate::SemanticsUpdate(SemanticsNodeUpdates nodes,

lib/ui/semantics/semantics_update.h

@@ -21,9 +21,9 @@ class SemanticsUpdate : public RefCountedDartWrappable<SemanticsUpdate> {
 
  public:
   ~SemanticsUpdate() override;
-  static fml::RefPtr<SemanticsUpdate> create(
-      SemanticsNodeUpdates nodes,
-      CustomAccessibilityActionUpdates actions);
+  static void create(Dart_Handle semantics_update_handle,
+                     SemanticsNodeUpdates nodes,
+                     CustomAccessibilityActionUpdates actions);
 
   SemanticsNodeUpdates takeNodes();
 

lib/ui/semantics/semantics_update_builder.cc

@@ -121,8 +121,9 @@ void SemanticsUpdateBuilder::updateCustomAction(int id,
   actions_[id] = action;
 }
 
-fml::RefPtr<SemanticsUpdate> SemanticsUpdateBuilder::build() {
-  return SemanticsUpdate::create(std::move(nodes_), std::move(actions_));
+void SemanticsUpdateBuilder::build(Dart_Handle semantics_update_handle) {
+  SemanticsUpdate::create(semantics_update_handle, std::move(nodes_),
+                          std::move(actions_));
 }
 
 }  // namespace flutter

lib/ui/semantics/semantics_update_builder.h

@@ -58,7 +58,7 @@ class SemanticsUpdateBuilder
                           std::string hint,
                           int overrideId);
 
-  fml::RefPtr<SemanticsUpdate> build();
+  void build(Dart_Handle semantics_update_handle);
 
   static void RegisterNatives(tonic::DartLibraryNatives* natives);