PolymerExpressions: fix for null iterators in comprehensions

pkg/polymer_expressions/test/eval_test.dart

Index: pkg/polymer_expressions/test/eval_test.dart
diff --git a/pkg/polymer_expressions/test/eval_test.dart b/pkg/polymer_expressions/test/eval_test.dart
index ece439689b91df395ea137acb8550be6ba7f45a9..6be36e46c0159ddbdbe9a3f7df34a6d9adb72bb6 100644
--- a/pkg/polymer_expressions/test/eval_test.dart
+++ b/pkg/polymer_expressions/test/eval_test.dart
@@ -197,7 +197,7 @@ main() {
       var scope = new Scope(variables: {'items': null});
       var comprehension = eval(parse('item in items'), scope);
       expect(comprehension, isNotNull);
-      expect(comprehension.iterable, null);
+      expect(comprehension.iterable, []]);
     });
 
   });