Roll Observatory packages and add a roll script

packages/usage/lib/src/usage_impl_io.dart

 // Copyright (c) 2014, 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.
 
 library usage_impl_io;
 
 import 'dart:async';
 import 'dart:convert' show JSON;
 import 'dart:io';
 
@@ skipping 67 matching lines @@
   void operator[]=(String key, dynamic value) {
     if (value == null) {
       _map.remove(key);
     } else {
       _map[key] = value;
     }
 
     _file.writeAsStringSync(JSON.encode(_map) + '\n');
   }
 }