Eliminates an extra copy when returning messages from the host platform to dart.

Author: gaaclarke

lib/ui/window/platform_message_response_dart.cc

@@ -42,8 +42,11 @@ 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());
+        size_t data_size = data->GetSize();
+        Dart_Handle byte_buffer = tonic::DartByteData::Adopt(
+            const_cast<uint8_t*>(data->GetMapping()), data_size);
+        // TODO(gaaclarke): fix the leak;
+        data.release();
         tonic::DartInvoke(callback.Release(), {byte_buffer});
       }));
 }

third_party/tonic/typed_data/dart_byte_data.cc

@@ -36,6 +36,11 @@ Dart_Handle DartByteData::Create(const void* data, size_t length) {
   }
 }
 
+Dart_Handle DartByteData::Adopt(void* data, size_t length) {
+  return Dart_NewExternalTypedDataWithFinalizer(
+      Dart_TypedData_kByteData, data, length, data, length, FreeFinalizer);
+}
+
 DartByteData::DartByteData()
     : data_(nullptr), length_in_bytes_(0), dart_handle_(nullptr) {}
 

third_party/tonic/typed_data/dart_byte_data.h

@@ -15,6 +15,8 @@ namespace tonic {
 class DartByteData {
  public:
   static Dart_Handle Create(const void* data, size_t length);
+  /// Takes ownership over [data] and calls [free] on it when it is done.
+  static Dart_Handle Adopt(void* data, size_t length);
 
   explicit DartByteData(Dart_Handle list);
   DartByteData(DartByteData&& other);