Allow null assignments if there is a corresponding marker.

tests/html/input_element_test.dart

Index: tests/html/input_element_test.dart
diff --git a/tests/html/input_element_test.dart b/tests/html/input_element_test.dart
index fe90511d817eaba3210741326d7171836e980759..43f245cf8156416d9e30be42f9f61db72b93ddf7 100644
--- a/tests/html/input_element_test.dart
+++ b/tests/html/input_element_test.dart
@@ -177,4 +177,12 @@ main() {
       check(new ButtonInputElement(), 'button');
     });
   });
+
+  group('attributes', () {
+    test('valueSetNull', () {
+      final e = new TextInputElement();
+      e.value = null;
+      expect(e.value, '');
+    });
+  });
 }