final fields are not written during serialization

### Source code
```dart
@JsonSerializable
class MyClass {
@JsonKey(name: 'value1')
var value1 = '';

@JsonKey(name: 'value2')
final value2 = 'must always be this';
}
```

### Generated code:
```dart
MyClass _$MyClassFromJson(Map json) {
return MyClass()..value1 = json['value1'] as String;
}

Map _$MyClassToJson(MyClass instance) => {
'value1': instance.value1,
};
```

### Actual result
vars are written, finals are not

### Expected result
all fields must be written - vars and finals

### Versions
```
Flutter 2.0.6
json_annotation: ^4.0.0
json_serializable: ^4.1.0
```