Possible bug relating to unused getters

```shell
$ flutter --version
Flutter 2.8.0 • channel stable • https://github.com/flutter/flutter.git
Framework • revision cf44000065 (9 days ago) • 2021-12-08 14:06:50 -0800
Engine • revision 40a99c5951
Tools • Dart 2.15.0
```

This is similar to issue #918 in that it is using extension of a base class, but is actually quite different. I have a situation where I have a base class (a business layer entity) which I am extending in the network layer in order to deserialise the server supplied JSON. The names of the properties in the server-side JSON is pretty verbose and I want to use shorter names in the business entity layer, I therefore need to find a way to tell `json_deserialize` to map the verbosely named json property into the entity property name.

Given the following:-
```dart
class Base {
final int name;

Base({required this.name});
}

@JsonSerializable()
class BaseModel extends Base {
BaseModel({required int reallyVeryVerboseName}) :
super(name: reallyVeryVerboseName);

int get reallyVeryVerboseName => name;

@override
@JsonKey(ignore: true)
int get name;

factory BaseModel.fromJson(Map json) =>
_$BaseModelFromJson(json);

}
```
it works flawlessly, e.g. I need to supply a getter for the verbose name and override the getter for the simple name and the following code is generated:-
```dart
BaseModel _$BaseModelFromJson(Map json) => BaseModel(
reallyVeryVerboseName: json['reallyVeryVerboseName'] as int,
);

Map _$BaseModelToJson(BaseModel instance) => {
'reallyVeryVerboseName': instance.reallyVeryVerboseName,
};
```
However, I don't need to serialise the `BaseModel` therefore added `createToJson: false` and expected to be able to remove the two getter methods, e.g.
```dart
class Base {
final int name;

Base({required this.name});
}

@JsonSerializable(createToJson: false)
class BaseModel extends Base {
BaseModel({required int reallyVeryVerboseName}) :
super(name: reallyVeryVerboseName);

factory BaseModel.fromJson(Map json) =>
_$BaseModelFromJson(json);
}
```
but when I do this I get the following exception:-
```shell
[SEVERE] json_serializable:json_serializable on lib/models.dart:

Cannot populate the required constructor argument: reallyVeryVerboseName.
package:test/models.dart:159:3
╷
9 │ BaseModel({required int reallyVeryVerboseName}) :
│ ^
╵
```
Leaving the two getter methods in place and adding `createToJson: false`, generates the following:-
```dart
BaseModel _$BaseModelFromJson(Map json) => BaseModel(
reallyVeryVerboseName: json['reallyVeryVerboseName'] as int,
);
```
So it seems that even when setting `createToJson: false` the getters are still required even though they are not used in the generated code.

So, Is this a bug? Or am I missing something fundamental?

I'm really very new to dart/flutter - I've only been looking for about a week so it may well just be lack of knowledge / understanding on my part.