Roll Observatory packages and add a roll script

packages/yaml/lib/src/yaml_document.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 yaml.yaml_document;
 
 import 'dart:collection';
 
 import 'package:source_span/source_span.dart';
 
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   /// The handle for use in the document.
   final String handle;
 
   /// The prefix that the handle maps to.
   final String prefix;
 
   TagDirective(this.handle, this.prefix);
 
   String toString() => "%TAG $handle $prefix";
 }