Added TodoMVC startup benchmarks.

samples/third_party/todomvc_performance/web/elements/td_item.html

 <link rel="import" href="td_input.html">
 <polymer-element name="td-item" extends="li" on-blur="{{commitAction}}">
   <template>
     <link rel="stylesheet" href="td_item.css">
     <div class="view {{ {'completed': item.completed, 'editing': editing} }}"
         hidden?="{{editing}}" on-dblclick="{{editAction}}">
       <input type="checkbox" class="toggle" checked="{{item.completed}}"
           on-click="{{itemChangeAction}}">
       <label>{{item.title}}</label>
       <button class="destroy" on-click="{{destroyAction}}"></button>
     </div>
     <input is="td-input" id="edit" class="edit" value="{{item.title}}"
         hidden?="{{!editing}}" on-td-input-commit="{{commitAction}}"
         on-td-input-cancel="{{cancelAction}}">
   </template>
   <script type="application/dart" src="td_item.dart"></script>
 </polymer-element>