Roll polymer packages to version 0.3.4

pkg/observe/lib/src/list_diff.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_diff;
 
 import 'dart:math' as math;
 import 'dart:collection' show UnmodifiableListView;
+import 'change_record.dart' show ChangeRecord;
 
 /// A summary of an individual change to a [List].
 ///
 /// Each delta represents that at the [index], [removed] sequence of items were
 /// removed, and counting forward from [index], [addedCount] items were added.
-class ListChangeRecord {
+class ListChangeRecord extends ChangeRecord {
   /// The list that changed.
   final List object;
 
   /// The index of the change.
   int get index => _index;
 
   /// The items removed, if any. Otherwise this will be an empty list.
   List get removed => _unmodifiableRemoved;
   UnmodifiableListView _unmodifiableRemoved;
 
@@ skipping 357 matching lines @@
       continue;
     }
 
     splices.addAll(calcSplices(list, splice.index,
         splice.index + splice.addedCount, splice._removed, 0,
         splice.removed.length));
   }
 
   return splices;
 }