Roll Observatory packages and add a roll script

packages/code_transformers/lib/assets.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.
 
 /// Common methods used by transfomers for dealing with asset IDs.
 library code_transformers.assets;
 
 import 'dart:math' show min, max;
 
 import 'package:barback/barback.dart';
@@ skipping 130 matching lines @@
     }
     return new Uri(
             path: path.url.relative(assetId.path, from: path.url.dirname(from.path)))
         .toString();
   }
 
   return Uri
       .parse('package:${assetId.package}/${assetId.path.substring(4)}')
       .toString();
 }