Roll Observatory packages and add a roll script

packages/observe/lib/src/list_path_observer.dart

 // Copyright (c) 2013, 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 observe.src.list_path_observer;
 
 import 'dart:async';
 import 'package:observe/observe.dart';
 
 // Inspired by ArrayReduction at:
@@ skipping 53 matching lines @@
         pathObs.open(_scheduleReduce);
         _observers.add(pathObs);
       }
     } else if (lengthAdjust < 0) {
       for (int i = 0; i < -lengthAdjust; i++) {
         _observers.removeLast().close();
       }
     }
   }
 }