Remove the crypto factories

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 6dd198ef74f6e3e809c74ceee60a065611bba01f..f1c20e3261d22580b69f2db0b3c255bf11fa3686 100644
--- a/pkg/crypto/lib/src/sha256.dart
+++ b/pkg/crypto/lib/src/sha256.dart
@@ -4,10 +4,12 @@
 
 part of crypto;
 
-// The SHA256 hasher is used to compute an SHA256 message digest.
-class _SHA256 extends _HashBase implements SHA256 {
+/**
+ * SHA256 hash function implementation.
+ */
+class SHA256 extends _HashBase {
   // Construct a SHA256 hasher object.
-  _SHA256() : _w = new List(64), super(16, 8, true) {
+  SHA256() : _w = new List(64), super(16, 8, true) {
     // Initial value of the hash parts. First 32 bits of the fractional parts
     // of the square roots of the first 8 prime numbers.
     _h[0] = 0x6a09e667;