pkg/crypto: move tests to unittest package

pkg/crypto/lib/src/sha256.dart

Index: pkg/crypto/lib/src/sha256.dart
diff --git a/pkg/crypto/lib/src/sha256.dart b/pkg/crypto/lib/src/sha256.dart
index b429b8f192e56b09b493042cfb87e62bc8271f99..88b701ecc0ea4d69071446e93c6c1f7fe8b6e2d7 100644
--- a/pkg/crypto/lib/src/sha256.dart
+++ b/pkg/crypto/lib/src/sha256.dart
@@ -8,6 +8,8 @@ part of crypto;
  * SHA256 hash function implementation.
  */
 class SHA256 extends _HashBase {
+  final List<int> _w;
+
   // Construct a SHA256 hasher object.
   SHA256() : _w = new List(64), super(16, 8, true) {
     // Initial value of the hash parts. First 32 bits of the fractional parts
@@ -102,6 +104,4 @@ class SHA256 extends _HashBase {
     _h[6] = _add32(g, _h[6]);
     _h[7] = _add32(h, _h[7]);
   }
-
-  final List<int> _w;
 }