Roll Observatory packages and add a roll script

packages/yaml/lib/src/equality.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.equality;
 
 import 'dart:collection';
 
 import 'package:collection/collection.dart';
 
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       } else {
         return value.hashCode;
       }
     } finally {
       parents.removeLast();
     }
   }
 
   return _deepHashCode(obj);
 }