Add paper-menu-button and its dependencies to third_party/polymer

third_party/polymer/v1_0/components/iron-pages/iron-pages.html

 <!--
 @license
 Copyright (c) 2015 The Polymer Project Authors. All rights reserved.
 This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
 The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
 The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
 Code distributed by Google as part of the polymer project is also
 subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 -->
 
@@ skipping 60 matching lines @@
       Polymer.IronSelectableBehavior
     ],
 
     properties: {
 
       // as the selected page is the only one visible, activateEvent
       // is both non-sensical and problematic; e.g. in cases where a user
       // handler attempts to change the page and the activateEvent
       // handler immediately changes it back
       activateEvent: {
+        type: String,
         value: null
       }
 
     },
 
     observers: [
       '_selectedPageChanged(selected)'
     ],
 
     _selectedPageChanged: function(selected, old) {
       this.async(this.notifyResize);
     }
   });
 
 </script>