Also test setting input field to null via undefined.

tests/html/input_element_test.dart

 library input_element_test;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_individual_config.dart';
 import 'dart:html';
 
 void check(InputElement element, String type, [bool supported = true]) {
   expect(element is InputElement, true);
   if (supported) {
     expect(element.type, type);
   } else {
@@ skipping 166 matching lines @@
       check(new ButtonInputElement(), 'button');
     });
   });
 
   group('attributes', () {
     test('valueSetNull', () {
       final e = new TextInputElement();
       e.value = null;
       expect(e.value, '');
     });
+    test('valueSetNullProxy', () {
+      final e = new TextInputElement();
+      var list = new List(5);
+      e.value = list[0];
+      expect(e.value, '');
+    });
   });
 }