Use patching for dart:io.

lib/io/base64.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 _Base64 {
-  static const List<int> _encodingTable = const [
+  static const List<String> _encodingTable = const [
       'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O',
       'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd',
       'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's',
       't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7',
       '8', '9', '+', '/'];
 
   /**
    * Base64 transfer encoding for MIME (RFC 2045)
    */
   static String _encode(List<int> data) {
@@ skipping 78 matching lines @@
         if (padCount == 0) {
           result.add(value & 0xFF);
         }
         charCount = 0;
         value = 0;
       }
     }
     return result;
   }
 }