Fix polymer_expression binding bug with observable list

pkg/polymer_expressions/lib/eval.dart

Index: pkg/polymer_expressions/lib/eval.dart
diff --git a/pkg/polymer_expressions/lib/eval.dart b/pkg/polymer_expressions/lib/eval.dart
index c33a262ad510831dbd9f5b56dc0c55a8bc8f18a4..a33abeb126709838ae4ed21109cce2528924f15c 100644
--- a/pkg/polymer_expressions/lib/eval.dart
+++ b/pkg/polymer_expressions/lib/eval.dart
@@ -650,7 +650,12 @@ class IndexObserver extends ExpressionObserver<Index> implements Index {
     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);