fix analyzer error

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 6ca92aba9771b6cf0049f14f04f26a550e228635..4501ef5dcb5484b0e52cb2325b8303727360408c 100644
--- a/pkg/polymer_expressions/lib/eval.dart
+++ b/pkg/polymer_expressions/lib/eval.dart
@@ -495,8 +495,8 @@ class BinaryObserver extends ExpressionObserver<BinaryOperator>
       _value = null;
     } else {
       if (operator == '|' && left._value is ObservableList) {
-        _subscription = left._value.listChanges.listen(
-            (_) => _invalidate(scope));
+        _subscription = (left._value as ObservableList).listChanges
+            .listen((_) => _invalidate(scope));
       }
       _value = f(left._value, right._value);
     }