dart:html perf optimization based on runing Dromaeo benchmarks

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

 #library("dom_attr_html");
 #import("dart:html");
 #import("dart:json");
 #source("Common.dart");
 #source("RunnerSuite.dart");
 
 void main() {
   final int num = 10240;
 
   // Try to force real results.
@@ skipping 14 matching lines @@
     .test('setAttribute', () {
         for (int i = 0; i < num; i++)
         a.attributes['id'] = 'foo';
     })
     .test('element.property = value', () {
       for (int i = 0; i < num; i++)
         a.id = 'foo';
     })
     .end();
 }