Make sure 'enumerate' changes when the underlying collection changes.

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 b774807d05d0665b57a0b69f230a5cce5e5b2a49..6ca92aba9771b6cf0049f14f04f26a550e228635 100644
--- a/pkg/polymer_expressions/lib/eval.dart
+++ b/pkg/polymer_expressions/lib/eval.dart
@@ -494,6 +494,10 @@ class BinaryObserver extends ExpressionObserver<BinaryOperator>
     } else if (left._value == null || right._value == null) {
       _value = null;
     } else {
+      if (operator == '|' && left._value is ObservableList) {
+        _subscription = left._value.listChanges.listen(
+            (_) => _invalidate(scope));
+      }
       _value = f(left._value, right._value);
     }
   }