Add Base64 codec to dart:convert.

sdk/lib/convert/convert_sources.gypi

 # Copyright (c) 2013, 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.
 
 # This file contains all sources for the dart:convert library.
 {
   'sources': [
     'convert.dart',
     # The above file needs to be first as it lists the parts below.
     'ascii.dart',
+    'base64.dart',
     'byte_conversion.dart',
     'chunked_conversion.dart',
     'codec.dart',
     'converter.dart',
     'encoding.dart',
     'html_escape.dart',
     'json.dart',
     'latin1.dart',
     'line_splitter.dart',
     'string_conversion.dart',
     'utf.dart',
   ],
 }