Use the new Base64 support from dart:convert in Observatory.

runtime/observatory/observatory_sources.gypi

 # Copyright (c) 2015, 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 dart, css, and html sources for Observatory.
 {
   'sources': [
     'lib/app.dart',
-    'lib/base64.dart',
     'lib/cli.dart',
     'lib/cpu_profile.dart',
     'lib/debugger.dart',
     'lib/elements.dart',
     'lib/elements.html',
     'lib/object_graph.dart',
     'lib/service.dart',
     'lib/service_common.dart',
     'lib/service_io.dart',
     'lib/service_html.dart',
@@ skipping 132 matching lines @@
     'lib/tracer.dart',
     'lib/utils.dart',
     'web/index.html',
     'web/main.dart',
     'web/favicon.ico',
     'web/third_party/trace_viewer_full.html',
     'web/timeline.js',
     'web/timeline.html',
   ],
 }