Eliminates an extra copy when returning messages from the host platfo…

…rm to dart.

lib/ui/window/platform_message_response_dart.cc

@@ -14,6 +14,12 @@
 
 namespace flutter {
 
+namespace {
+void MappingFinalizer(void* isolate_callback_data, void* peer) {
+  delete static_cast<fml::Mapping*>(peer);
+}
+}  // anonymous namespace
+
 PlatformMessageResponseDart::PlatformMessageResponseDart(
     tonic::DartPersistentValue callback,
     fml::RefPtr<fml::TaskRunner> ui_task_runner)
@@ -42,8 +48,15 @@ void PlatformMessageResponseDart::Complete(std::unique_ptr<fml::Mapping> data) {
         }
         tonic::DartState::Scope scope(dart_state);
 
-        Dart_Handle byte_buffer =
-            tonic::DartByteData::Create(data->GetMapping(), data->GetSize());
+        void* mapping = const_cast<uint8_t*>(data->GetMapping());
+        size_t data_size = data->GetSize();
+        Dart_Handle byte_buffer = Dart_NewExternalTypedDataWithFinalizer(
+            /*type=*/Dart_TypedData_kByteData,
+            /*data=*/mapping,
+            /*length=*/data_size,
+            /*peer=*/data.release(),
+            /*external_allocation_size=*/data_size,
+            /*callback=*/MappingFinalizer);
         tonic::DartInvoke(callback.Release(), {byte_buffer});
       }));
 }