dart:html perf optimization based on runing Dromaeo benchmarks

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

 #library("dom_modify_html");
 #import("dart:html");
 #import("dart:json");
 #source("Common.dart");
 #source("RunnerSuite.dart");
 
 void main() {
   final int num = 400;
 
   String str = 'null';
@@ skipping 48 matching lines @@
     .test('appendChild', () {
       for (int i = 0; i < elems.length; i++)
         document.body.nodes.add(elems[i]);
     })
     .test('insertBefore', () {
       for (int i = 0; i < elems.length; i++)
         document.body.insertBefore(elems[i], document.body.nodes.first);
     })
     .end();
 }