dart:html perf optimization based on runing Dromaeo benchmarks

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

 #library("dom_traverse_html");
 #import("dart:html");
 #import('../common/common.dart');
 #source("Common.dart");
 #source("RunnerSuite.dart");
 
 void main() {
   final int num = 40;
 
   // Try to force real results.
@@ skipping 67 matching lines @@
     .test('childNodes', () {
       for (int i = 0; i < num; i++) {
         final nodes = document.body.nodes;
         for (int j = 0; j < nodes.length; j++) {
           ret = nodes[j];
         }
       }
     })
     .end();
 }