.freeze() still allows modifications, from e.g. .mergeFromProto3Json()

[azurvii]

For example, the following code would not trigger a runtime exception:

MyMessage.create()
    ..mergeFromProto3Json(json.decode('{"id":888}'))
    ..freeze()
    ..mergeFromProto3Json(json.decode('{"id":999}'));

Or:

MyMessage.getDefault()
    ..mergeFromProto3Json(json.decode('{"id":999}'));

The second would change the singleton default instance, which, according to the code generated for me, is a frozen object.

My setup:

$ flutter --version
Flutter 2.0.2 • channel stable • https://github.com/flutter/flutter.git
Framework • revision 8962f6dc68 (5 days ago) • 2021-03-11 13:22:20 -0800
Engine • revision 5d8bf811b3
Tools • Dart 2.12.1
$ protoc --version
libprotoc 3.15.6
$ flutter pub global list
protoc_plugin 20.0.0