[cryptography_flutter] Fix platform conditions #228

Job	Run time
Analyze (cryptography_flutter, 3.35.0)	47s
Analyze (cryptography_flutter/example, beta)	46s
Analyze (cryptography_flutter/example, 3.35.0)	48s
Analyze (cryptography, 3.3.0)	43s
Analyze (cryptography_flutter_integration_test, beta)	50s
Analyze (cryptography_flutter_integration_test, 3.35.0)	52s
Analyze (cryptography_test, beta)	27s
Analyze (cryptography_test, 3.3.0)	49s
Analyze (cryptography, beta)	30s
Analyze (cryptography_flutter, beta)	48s
Analyze (jwk, 3.3.0)	36s
Analyze (jwk, beta)	33s
PANA (jwk)	1m 51s
PANA (cryptography)	1m 56s
Test (cryptography, vm)	46s
Test (cryptography, dart2wasm)	2m 40s
Test (jwk, dart2wasm)	31s
Test (jwk, vm)	21s
Test (cryptography, dart2js)	2m 43s
Test (jwk, dart2js)	45s
PANA	0s
	20m 2s

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