Fix migration for freezed classes #266

Job	Run time
check-score (hive)	1m 31s
ensure-codegen	2m 54s
ensure-embedme	18s
check-score (hive_flutter)	2m 21s
code-quality (hive, stable)	1m 21s
check-score (hive_generator)	2m 32s
code-quality (hive, beta)	1m 15s
test-hive (vm, kernel)	57s
test-hive_flutter (stable)	51s
test-hive (chrome, dart2js)	3m 51s
code-quality (hive_flutter, stable)	1m 35s
test-hive_generator (stable, ubuntu-latest)	3m 51s
test-hive_flutter (beta)	56s
test-hive_generator (stable, windows-latest)	3m 42s
test-hive (chrome, dart2wasm)	3m 10s
code-quality (hive_flutter, beta)	1m 24s
test-hive_generator (beta, ubuntu-latest)	3m 50s
code-quality (hive_generator, stable)	1m 24s
code-quality (hive_generator, beta)	1m 13s
	38m 56s

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