Change bytes accessors to expose only unmodifiable List<int>, making Bytes as a type an internal implementation detail.

PiperOrigin-RevId: 580290313

conformance/conformance_dart.dart

@@ -1,6 +1,5 @@
 import 'dart:io';
 
-import 'package:pb_runtime/ffi/bytes.dart';
 import 'package:pb_runtime/pb_runtime.dart' as pb;
 import 'package:third_party.protobuf/test_messages_proto2.upb.dart';
 import 'package:third_party.protobuf/test_messages_proto3.upb.dart';
@@ -50,8 +49,8 @@ ConformanceResponse doTest(ConformanceRequest request) {
     case ConformanceRequest_payload.protobufPayload:
       try {
         testMessage = isProto3
-            ? TestAllTypesProto3.fromBinary(request.protobufPayload.data)
-            : TestAllTypesProto2.fromBinary(request.protobufPayload.data);
+            ? TestAllTypesProto3.fromBinary(request.protobufPayload)
+            : TestAllTypesProto2.fromBinary(request.protobufPayload);
       } catch (e) {
         final parseErrorResponse = ConformanceResponse();
         parseErrorResponse.parseError = '$e';
@@ -66,8 +65,7 @@ ConformanceResponse doTest(ConformanceRequest request) {
   switch (request.requestedOutputFormat) {
     case WireFormat.PROTOBUF:
       try {
-        response.protobufPayload =
-            Bytes(pb.GeneratedMessage.toBinary(testMessage));
+        response.protobufPayload = pb.GeneratedMessage.toBinary(testMessage);
       } catch (e) {
         response.serializeError = '$e';
       }