[Dart] Add missing output @JsonKey output for @freezed option #605

Job	Run time
build	43s
typescript,typescript-zod,typescript-effect-schema	3m 18s
javascript,schema-javascript	1m 9s
golang,schema-golang	1m 30s
cjson,schema-cjson	2m 50s
cplusplus,schema-cplusplus	4m 55s
flow,schema-flow	2m 18s
java,schema-java	10m 8s
python,schema-python	2m 36s
haskell,schema-haskell	11m 39s
csharp,schema-csharp,schema-json-csharp,graphql-csharp,csharp-SystemTextJson	9m 34s
json-ts-csharp	2m 20s
dart,schema-dart	1m 53s
swift,schema-swift	4m 9s
javascript-prop-types	1m 38s
ruby	1m 21s
php	1m 2s
scala3,schema-scala3	2m 46s
rust,schema-rust	4m 6s
kotlin,schema-kotlin,kotlin-jackson,schema-kotlin-jackson	6m 18s
objective-c	2m 51s
test-complete	1s
	1h 19m 5s

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