Make interfaces into abstract classes in 'dart:crypto'.

sdk/lib/crypto/crypto.dart

Index: sdk/lib/crypto/crypto.dart
diff --git a/sdk/lib/crypto/crypto.dart b/sdk/lib/crypto/crypto.dart
index ff634928d102fc2db377eac627e68978c0664820..c8f9a0f5a9e724e5ce7ce4e37fc24b31cd9306ae 100644
--- a/sdk/lib/crypto/crypto.dart
+++ b/sdk/lib/crypto/crypto.dart
@@ -52,15 +52,15 @@ abstract class Hash {
 /**
  * SHA1 hash function implementation.
  */
-interface SHA1 extends Hash default _SHA1 {
-  SHA1();
+abstract class SHA1 implements Hash {
+  factory SHA1() => new _SHA1();
 }
 
 /**
  * SHA256 hash function implementation.
  */
-interface SHA256 extends Hash default _SHA256 {
-  SHA256();
+abstract class SHA256 implements Hash {
+  factory SHA256() => new _SHA256();
 }
 
 /**
@@ -69,8 +69,8 @@ interface SHA256 extends Hash default _SHA256 {
  * WARNING: MD5 has known collisions and should only be used when
  * required for backwards compatibility.
  */
-interface MD5 extends Hash default _MD5 {
-  MD5();
+abstract class MD5 implements Hash {
+  factory MD5() => new _MD5();
 }
 
 /**
@@ -79,11 +79,11 @@ interface MD5 extends Hash default _MD5 {
  * The [update] method is used to add data to the message. The [digest] method
  * is used to extract the message authentication code.
  */
-interface HMAC default _HMAC {
+abstract class HMAC {
   /**
    * Create an [HMAC] object from a [Hash] and a key.
    */
-  HMAC(Hash hash, List<int> key);
+  factory HMAC(Hash hash, List<int> key) => new _HMAC();
 
   /**
    * Add a list of bytes to the message.
@@ -100,7 +100,7 @@ interface HMAC default _HMAC {
 /**
  * Utility methods for working with message digests.
  */
-class CryptoUtils {
+abstract class CryptoUtils {
   /**
    * Convert a list of bytes (for example a message digest) into a hex
    * string.