update elements and fix some bugs

lib/src/iron-meta/iron-meta.html

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 @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
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 -->
 
@@ skipping 40 matching lines @@
 @element iron-meta
 -->
 
 <script>
 
   (function() {
 
     // monostate data
     var metaDatas = {};
     var metaArrays = {};
+    var singleton = null;
 
     Polymer.IronMeta = Polymer({
 
       is: 'iron-meta',
 
       properties: {
 
         /**
          * The type of meta-data.  All meta-data of the same type is stored
          * together.
@@ skipping 32 matching lines @@
         /**
          * Array of all meta-data values for the given type.
          */
         list: {
           type: Array,
           notify: true
         }
 
       },
 
+      hostAttributes: {
+        hidden: true
+      },
+
       /**
        * Only runs if someone invokes the factory/constructor directly
        * e.g. `new Polymer.IronMeta()`
        */
       factoryImpl: function(config) {
         if (config) {
           for (var n in config) {
             switch(n) {
               case 'type':
               case 'key':
@@ skipping 74 matching lines @@
           if (key in data) {
             var value = data[key];
             delete data[key];
             this.arrayDelete(list, value);
           }
         }
       }
 
     });
 
+    Polymer.IronMeta.getIronMeta = function getIronMeta() {
+       if (singleton === null) {
+         singleton = new Polymer.IronMeta();
+       }
+       return singleton;
+     };
+
     /**
     `iron-meta-query` can be used to access infomation stored in `iron-meta`.
 
     Examples:
 
     If I create an instance like this:
 
         <iron-meta key="info" value="foo/bar"></iron-meta>
 
     Note that value="foo/bar" is the metadata I've defined. I could define more
@@ skipping 91 matching lines @@
        * @return {*}
        */
       byKey: function(key) {
         return this._metaData && this._metaData[key];
       }
 
     });
 
   })();
 </script>