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

sdk/lib/crypto/crypto_utils.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.
 
 class _LineWrappingStringBuffer {
   _LineWrappingStringBuffer(int this._lineLength) : _sb = new StringBuffer();
 
   void add(String s) {
     if (_lineLength != null && _currentLineLength == _lineLength) {
       _sb.add('\r\n');
       _currentLineLength = 0;
     }
     _sb.add(s);
     _currentLineLength++;
   }
 
   String toString() => _sb.toString();
 
   int _lineLength;
   StringBuffer _sb;
   int _currentLineLength = 0;
 }
 
-class _CryptoUtils {
+abstract class _CryptoUtils {
   static String bytesToHex(List<int> bytes) {
     var result = new StringBuffer();
     for (var part in bytes) {
       result.add('${part < 16 ? '0' : ''}${part.toRadixString(16)}');
     }
     return result.toString();
   }
 
   static String bytesToBase64(List<int> bytes, [int lineLength]) {
     final table =
@@ skipping 30 matching lines @@
       var b1 = bytes[i + 1] & 0xff;
       result.add(table[b0 >> 2]);
       result.add(table[((b0 << 4) | (b1 >> 4)) & 0x3f]);
       result.add(table[(b1 << 2) & 0x3f]);
       result.add('=');
     }
 
     return result.toString();
   }
 }