ParentNode#appendChild(null) shouldn't crash

sky/tests/dom/appendChild.sky

 <sky>
 <import src="../resources/dom-utils.sky" as="DomUtils" />
 <script>
 import "../resources/third_party/unittest/unittest.dart";
 import "../resources/unit.dart";
 
 import "dart:sky";
 
 void main() {
   initUnit();
 
   var childElementCount = DomUtils.childElementCount;
   var childNodeCount = DomUtils.childNodeCount;
 
   test("should throw with invalid arguments", () {
     var parent = document.createElement("div");
     expect(() {
       parent.appendChild();
     }, throws);
-    // TODO(dart): This is a real bug.
-    // expect(() {
-    //   parent.appendChild(null);
-    // }, throws);
+    expect(() {
+      parent.appendChild(null);
+    }, throws);
     expect(() {
       parent.appendChild({tagName: "div"});
     }, throws);
   });
 
   test("should insert children", () {
     var parent = document.createElement("div");
     var child1 = parent.appendChild(document.createElement("div"));
     var child2 = parent.appendChild(new Text(" text "));
     var child3 = parent.appendChild(new Text(" "));
@@ skipping 41 matching lines @@
   // TODO(dart): These might be real bugs too.
   // test("should throw when appending to a text", () {
   //   var parent = new Text();
   //   expect(() {
   //     parent.appendChild(document.createElement("div"));
   //   }, throws);
   // });
 }
 </script>
 </sky>