Changed interfaces to abstract classes where possible in {lib,samples,pkg}

lib/crypto/crypto.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #library('dart:crypto');
 
 #import('dart:math');
 
 #source('crypto_utils.dart');
 #source('hash_utils.dart');
 #source('hmac.dart');
 #source('md5.dart');
 #source('sha1.dart');
 #source('sha256.dart');
 
 /**
  * Interface for cryptographic hash functions.
  *
  * The [update] method is used to add data to the hash. The [digest] method
  * is used to extract the message digest.
  *
  * Once the [digest] method has been called no more data can be added using the
  * [update] method. If [update] is called after the first call to [digest] a
  * HashException is thrown.
  *
  * If multiple instances of a given Hash is needed the [newInstance]
  * method can provide a new instance.
  */
-interface Hash {
+abstract class Hash {
   /**
    * Add a list of bytes to the hash computation.
    */
   Hash update(List<int> data);
 
   /**
    * Finish the hash computation and extract the message digest as
    * a list of bytes.
    */
   List<int> digest();
@@ skipping 82 matching lines @@
 /**
  * HashExceptions are thrown on invalid use of a Hash
  * object.
  */
 class HashException {
   HashException(String this.message);
   toString() => "HashException: $message";
   String message;
 }