Remove the crypto factories

pkg/crypto/lib/src/md5.dart

Index: pkg/crypto/lib/src/md5.dart
diff --git a/pkg/crypto/lib/src/md5.dart b/pkg/crypto/lib/src/md5.dart
index 279db3d0b72a2952b619dd8e1ae7222c6dce16c6..6acf384c62278257208084f2b5a3d435725dded5 100644
--- a/pkg/crypto/lib/src/md5.dart
+++ b/pkg/crypto/lib/src/md5.dart
@@ -4,9 +4,14 @@
 
 part of crypto;
 
-// The MD5 hasher is used to compute an MD5 message digest.
-class _MD5 extends _HashBase implements MD5 {
-  _MD5() : super(16, 4, false) {
+/**
+ * MD5 hash function implementation.
+ *
+ * WARNING: MD5 has known collisions and should only be used when
+ * required for backwards compatibility.
+ */
+class MD5 extends _HashBase {
+  MD5() : super(16, 4, false) {
     _h[0] = 0x67452301;
     _h[1] = 0xefcdab89;
     _h[2] = 0x98badcfe;