Roll Observatory packages and add a roll script

packages/observe/lib/src/observable_list.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.observable_list;
 
 import 'dart:async';
 import 'dart:collection' show ListBase, UnmodifiableListView;
 import 'package:observe/observe.dart';
 import 'list_diff.dart' show projectListSplices, calcSplices;
@@ skipping 284 matching lines @@
 
     for (var change in changeRecords) {
       int addEnd = change.index + change.addedCount;
       int removeEnd = change.index + change.removed.length;
 
       var addedItems = current.getRange(change.index, addEnd);
       previous.replaceRange(change.index, removeEnd, addedItems);
     }
   }
 }