Silently fail when assigning to properties on null

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 213f17ba48deeeee732ade8a155f446cd9b43ec3..bdbbde5a6073d7af3a78f1644081501138d4f3ac 100644
--- a/pkg/polymer_expressions/test/eval_test.dart
+++ b/pkg/polymer_expressions/test/eval_test.dart
@@ -257,6 +257,10 @@ main() {
       expect(foo.name, '19');
     });
 
+    test('should throw on assignments to properties on null', () {
+      assign(parse('name'), 'b', new Scope(model: null));
+    });
+
   });
 
   group('scope', () {