Improve the style of tests.

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Review URL: https://codereview.chromium.org//1350553004 .

Author: nex3

pkgs/crypto/lib/crypto.dart

@@ -8,6 +8,7 @@ export 'src/base64.dart';
 export 'src/base64/decoder.dart';
 export 'src/base64/encoder.dart';
 export 'src/crypto_utils.dart';
+export 'src/hash.dart';
 export 'src/hmac.dart';
 export 'src/md5.dart';
 export 'src/sha1.dart';