dart:html perf optimization based on runing Dromaeo benchmarks

samples/third_party/dromaeo/tests/dom-modify-html.dart

Index: samples/third_party/dromaeo/tests/dom-modify-html.dart
diff --git a/samples/third_party/dromaeo/tests/dom-modify-html.dart b/samples/third_party/dromaeo/tests/dom-modify-html.dart
index d46cd477758130128548d9440fc0f296dfe388b4..a6a48b33313c9038773729500257aa7e5df4f76d 100644
--- a/samples/third_party/dromaeo/tests/dom-modify-html.dart
+++ b/samples/third_party/dromaeo/tests/dom-modify-html.dart
@@ -58,11 +58,11 @@ void main() {
     })
     .test('appendChild', () {
       for (int i = 0; i < elems.length; i++)
-        document.body.nodes.add(elems[i]);
+        document.body.$dom_appendChild(elems[i]);
     })
     .test('insertBefore', () {
       for (int i = 0; i < elems.length; i++)
-        document.body.insertBefore(elems[i], document.body.nodes.first);
+        document.body.insertBefore(elems[i], document.body.$dom_firstChild);
     })
     .end();
 }