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

sdk/lib/crypto/hash_utils.dart

Index: sdk/lib/crypto/hash_utils.dart
diff --git a/sdk/lib/crypto/hash_utils.dart b/sdk/lib/crypto/hash_utils.dart
index bce8c8ae2f95367f5bad538ea8ade120f362f768..d3c23f56840a419e901e80de5679252a6bc1eb2b 100644
--- a/sdk/lib/crypto/hash_utils.dart
+++ b/sdk/lib/crypto/hash_utils.dart
@@ -19,7 +19,7 @@ int _rotl32(int val, int shift) {
 
 // Base class encapsulating common behavior for cryptographic hash
 // functions.
-class _HashBase implements Hash {
+abstract class _HashBase implements Hash {
   _HashBase(int this._chunkSizeInWords,
             int this._digestSizeInWords,
             bool this._bigEndianWords)
@@ -58,10 +58,10 @@ class _HashBase implements Hash {
   }
 
   // Create a fresh instance of this Hash.
-  abstract newInstance();
+  newInstance();
 
   // One round of the hash computation.
-  abstract _updateHash(List<int> m);
+  _updateHash(List<int> m);
 
   // Helper methods.
   _add32(x, y) => (x + y) & _MASK_32;