Added TodoMVC startup benchmarks.

samples/third_party/todomvc_performance/web/elements/td_model.dart

 library todomvc.web.elements.td_model;
 
 import 'dart:html';
 import 'package:polymer/polymer.dart';
 import '../lib-elements/polymer_localstorage.dart';
 
 class Todo extends Observable {
   @observable String title;
   @observable bool completed = false;
 
@@ skipping 81 matching lines @@
     items.removeWhere(filters['completed']);
   }
 
   void setItemsCompleted(bool completed) {
     for (var item in items) {
       item.completed = completed;
     }
     itemsChanged();
   }
 }