Also test setting input field to null via undefined.

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 02fb0627e129fe5ed9cdcc28672c9cff62cfe7e1..0a12d0ba642299bc240f3ab506a179473bb722e2 100644
--- a/tests/html/input_element_test.dart
+++ b/tests/html/input_element_test.dart
@@ -184,5 +184,11 @@ main() {
       e.value = null;
       expect(e.value, '');
     });
+    test('valueSetNullProxy', () {
+      final e = new TextInputElement();
+      var list = new List(5);
+      e.value = list[0];
+      expect(e.value, '');
+    });
   });
 }