Implement md5 hash in crypto library.

lib/crypto/crypto.dart

Index: lib/crypto/crypto.dart
diff --git a/lib/crypto/crypto.dart b/lib/crypto/crypto.dart
index d34aeaac9efc4a23a84dfae8096a8dff86a66be0..7f193c6451aef9b931f059df05084a3e71c945af 100644
--- a/lib/crypto/crypto.dart
+++ b/lib/crypto/crypto.dart
@@ -5,8 +5,9 @@
 #library('crypto');
 
 #source('crypto_utils.dart');
+#source('hash_utils.dart');
 #source('hmac.dart');
-#source('sha_utils.dart');
+#source('md5.dart');
 #source('sha1.dart');
 #source('sha256.dart');
 
@@ -61,6 +62,16 @@ interface SHA256 extends Hash default _SHA256 {
 }
 
 /**
+ * MD5 hash function implementation.
+ *
+ * WARNING: MD5 has known collisions and should only be used when
+ * required for backwards compatibility.
+ */
+interface MD5 extends Hash default _MD5 {
+  MD5();
+}
+
+/**
  * Hash-based Message Authentication Code support.
  *
  * The [update] method is used to add data to the message. The [digest] method