Fix deprecation warnings in dart:io. Now completely warning free.

sdk/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.
 
 part of dart.io;
 
 class _Base64 {
   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',
@@ skipping 33 matching lines @@
       for (int j = 0; j < 2; j++) {
         int index = (value >> ((3 - j) * 6)) & ((1 << 6) - 1);
         characters.add(_encodingTable[index]);
       }
       characters.add("=");
       characters.add("=");
     }
     StringBuffer output = new StringBuffer();
     for (i = 0; i < characters.length; i++) {
       if (i > 0 && i % 76 == 0) {
-        output.add("\r\n");
+        output.write("\r\n");
       }
-      output.add(characters[i]);
+      output.write(characters[i]);
     }
     return output.toString();
   }
 
 
   /**
    * Base64 transfer decoding for MIME (RFC 2045).
    */
   static List<int> _decode(String data) {
     List<int> result = new List<int>();
@@ skipping 30 matching lines @@
         if (padCount == 0) {
           result.add(value & 0xFF);
         }
         charCount = 0;
         value = 0;
       }
     }
     return result;
   }
 }