Deprecating Element.elements for Element.children.

tests/html/hidden_dom_2_test.dart

 library HiddenDom2Test;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 // Test that the dart:html API does not leak native jsdom methods:
 //   appendChild operation.
 
 main() {
   useHtmlConfiguration();
 
   test('test1', () {
-    document.body.elements.add(new Element.html(r'''
+    document.body.children.add(new Element.html(r'''
 <div id='div1'>
 Hello World!
 </div>'''));
     Element e = document.query('#div1');
     Element e2 = new Element.html(r"<div id='xx'>XX</div>");
     expect(e, isNotNull);
 
     checkNoSuchMethod(() { confuse(e).appendChild(e2); });
 
   });
@@ skipping 15 matching lines @@
     action();
   } catch (e) {
     threw = true;
     ex = e;
   }
   if (!threw)
     expect(false, isTrue, reason: 'Action should have thrown exception');
 
   expect(ex, isNoSuchMethodError);
 }