Replaced Element.append with Node.appendChild

tests/html/node_test.dart

Index: tests/html/node_test.dart
diff --git a/tests/html/node_test.dart b/tests/html/node_test.dart
index c0a0dde4e607b5eec9c879aad8112f8ccc1a960f..e56403dcdf50eb438d6c1df02ddb04ec02011bb7 100644
--- a/tests/html/node_test.dart
+++ b/tests/html/node_test.dart
@@ -39,6 +39,12 @@ main() {
       expect(node.nodes[2], isComment);
     });
 
+    test('append', () {
+      var node = makeNode();
+      node.append(new Element.tag('hr'));
+      expect(node.nodes.last, isHRElement);
+    });
+
     test('remove', () {
       final node = makeNodeWithChildren();
       final subnode = node.nodes[1];