[Dart] fix: missing description emit when freezed is used #719

Job	Run time
build	28s
typescript,typescript-zod,typescript-effect-schema	1m 35s
javascript,schema-javascript	53s
golang,schema-golang	1m 14s
cjson,schema-cjson	1m 26s
cplusplus,schema-cplusplus	1m 35s
flow,schema-flow	1m 35s
java,schema-java	1m 35s
python,schema-python	1m 33s
haskell,schema-haskell	1m 34s
csharp,schema-csharp,schema-json-csharp,graphql-csharp,csharp-SystemTextJson	1m 35s
json-ts-csharp	1m 34s
dart,schema-dart	1m 15s
swift,schema-swift	1m 33s
javascript-prop-types	1m 13s
ruby	58s
php	53s
scala3,schema-scala3	1m 33s
rust,schema-rust	1m 30s
kotlin,schema-kotlin,kotlin-jackson,schema-kotlin-jackson	1m 32s
objective-c	1m 11s
test-complete	0s
	28m 15s

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