Have the same behavior for setting a CSS style to null versus empty string in all browsers.

tests/html/cssstyledeclaration_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library CssStyleDeclarationTest;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 import 'dart:async';
 import 'utils.dart';
@@ skipping 37 matching lines @@
       equals("color: blue;"));
   });
 
   test('CSS property empty getters and setters', () {
     var style = createTestStyle();
     expect(style.border, equals(""));
 
     style.border = "1px solid blue";
     style.border = "";
     expect(style.border, equals(""));
+
+    style.border = "1px solid blue";
+    style.border = null;
+    expect(style.border, equals(""));
   });
 
   test('CSS property getters and setters', () {
     var style = createTestStyle();
     expect(style.color, equals("blue"));
     expect(style.width, equals("2px"));
 
     style.color = "red";
     style.transform = "translate(10px, 20px)";
 
@@ skipping 72 matching lines @@
     expect(elements.style.backgroundColor, equals('green'));
     elements = document.queryAll('#wat');
     expect(elements.style.backgroundColor, equals('yellow'));
 
     elements.style.borderLeftWidth = '18px';
     expect(elements.style.borderLeftWidth, equals('18px'));
     elements = document.queryAll('li');
     expect(elements.style.borderLeftWidth, equals('10px'));
   });
 }