[cryptography_flutter] Fix platform conditions #230

Job	Run time
Analyze (cryptography_flutter, 3.35.0)	50s
Analyze (cryptography, beta)	36s
Analyze (cryptography, 3.3.0)	44s
Analyze (jwk, 3.3.0)	37s
Analyze (jwk, beta)	32s
Analyze (cryptography_test, beta)	34s
Analyze (cryptography_flutter/example, beta)	1m 4s
Analyze (cryptography_flutter, beta)	49s
Analyze (cryptography_flutter/example, 3.35.0)	51s
Analyze (cryptography_flutter_integration_test, 3.35.0)	1m 2s
Analyze (cryptography_flutter_integration_test, beta)	48s
Analyze (cryptography_test, 3.3.0)	41s
PANA (cryptography)	1m 54s
PANA (jwk)	2m 2s
Test (jwk, dart2js)	1m 50s
Test (cryptography, vm)	53s
Test (cryptography, dart2js)	3m 29s
Test (jwk, vm)	20s
Test (cryptography, dart2wasm)	3m 22s
Test (jwk, dart2wasm)	32s
PANA (cryptography_flutter)	1m 47s
	25m 17s

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