Roll Observatory packages and add a roll script

packages/code_transformers/lib/src/delete_file.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.
 
 /// Transformer that deletes everything that it sees, but only in release mode.
 library code_transformers.src.delete_file;
 
 import 'package:barback/barback.dart';
 
 // Deletes all files supplied in release mode.
 class DeleteFile extends Transformer {
   BarbackSettings settings;
 
   DeleteFile.asPlugin(this.settings);
 
   /// Only apply to files in release mode.
   isPrimary(_) => settings.mode == BarbackMode.RELEASE;
 
   apply(Transform transform) {
     transform.consumePrimary();
   }
 }