use .append instead of .nodes.add

tests/html/cross_frame_test.dart

 library CrossFrameTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 main() {
   useHtmlConfiguration();
 
   var isWindowBase = predicate((x) => x is WindowBase, 'is a WindowBase');
   var isWindow = predicate((x) => x is Window, 'is a Window');
   var isLocationBase = predicate((x) => x is LocationBase, 'is a LocationBase');
   var isLocation =
       predicate((x) => x is Location, 'is a Location');
   var isHistoryBase = predicate((x) => x is HistoryBase, 'is a HistoryBase');
   var isHistory = predicate((x) => x is History, 'is a History');
 
   final iframe = new Element.tag('iframe');
-  document.body.nodes.add(iframe);
+  document.body.append(iframe);
 
   test('window', () {
       expect(window, isWindow);
       expect(window.document, document);
     });
 
   test('iframe', () {
       final frameWindow = iframe.contentWindow;
       expect(frameWindow, isWindowBase);
       //TODO(gram) The next test should be written as:
@@ skipping 37 matching lines @@
 
       // Valid methods.
       frameHistory.forward();
 
       expect(() => frameHistory.length, throws);
 
       final frameParentHistory = iframe.contentWindow.parent.history;
       expect(frameParentHistory, isHistory);
     });
 }