Fix polymer_expression binding bug with observable list

pkg/polymer_expressions/lib/eval.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 polymer_expressions.eval;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:observe/observe.dart';
@@ skipping 632 matching lines @@
 
   _updateSelf(Scope scope) {
     var receiverValue = receiver._value;
     if (receiverValue == null) {
       _value = null;
       return;
     }
     var key = argument._value;
     _value = receiverValue[key];
 
-    if (receiverValue is Observable) {
+    if (receiverValue is ObservableList) {
+      _subscription = (receiverValue as ObservableList).listChanges
+          .listen((changes) {
+        if (changes.any((c) => c.indexChanged(key))) _invalidate(scope);
+      });
+    } else if (receiverValue is Observable) {
       _subscription = (receiverValue as Observable).changes.listen((changes) {
         if (changes.any((c) => c is MapChangeRecord && c.key == key)) {
           _invalidate(scope);
         }
       });
     }
   }
 
   accept(Visitor v) => v.visitIndex(this);
 }
@@ skipping 81 matching lines @@
 
   Comprehension(this.identifier, Iterable iterable)
       : iterable = (iterable != null) ? iterable : const [];
 }
 
 class EvalException implements Exception {
   final String message;
   EvalException(this.message);
   String toString() => "EvalException: $message";
 }