Roll polymer packages to version 0.3.4

pkg/observe/lib/src/list_diff.dart

Index: pkg/observe/lib/src/list_diff.dart
diff --git a/pkg/observe/lib/src/list_diff.dart b/pkg/observe/lib/src/list_diff.dart
index 59a969883796b4626cc6e3b27aa964a978685250..cbc3f2f8ed413f4a25b4dc8ab698b739ffdc6be3 100644
--- a/pkg/observe/lib/src/list_diff.dart
+++ b/pkg/observe/lib/src/list_diff.dart
@@ -6,12 +6,13 @@ 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;