update elements and fix some bugs

lib/src/iron-collapse/iron-collapse.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 144 matching lines @@
       this.style.transitionDuration = (enabled && this._enableTransition) ? '' : '0s';
     },
 
     _horizontalChanged: function() {
       this.dimension = this.horizontal ? 'width' : 'height';
       this.style.transitionProperty = this.dimension;
     },
 
     _openedChanged: function() {
       if (this.opened) {
+        this.setAttribute('aria-expanded', 'true');
+        this.setAttribute('aria-hidden', 'false');
+
         this.toggleClass('iron-collapse-closed', false);
         this.updateSize('auto', false);
         var s = this._calcSize();
         this.updateSize('0px', false);
         // force layout to ensure transition will go
         /** @suppress {suspiciousCode} */ this.offsetHeight;
         this.updateSize(s, true);
-      }
-      else {
+        // focus the current collapse
+        this.focus();
+      } else {
+        this.setAttribute('aria-expanded', 'false');
+        this.setAttribute('aria-hidden', 'true');
+
         this.toggleClass('iron-collapse-opened', false);
         this.updateSize(this._calcSize(), false);
         // force layout to ensure transition will go
         /** @suppress {suspiciousCode} */ this.offsetHeight;
         this.updateSize('0px', true);
       }
-      this.setAttribute('aria-expanded', this.opened ? 'true' : 'false');
-
     },
 
     _transitionEnd: function() {
       if (this.opened) {
         this.updateSize('auto', false);
       }
       this.toggleClass('iron-collapse-closed', !this.opened);
       this.toggleClass('iron-collapse-opened', this.opened);
       this.enableTransition(false);
     },
 
     _calcSize: function() {
       return this.getBoundingClientRect()[this.dimension] + 'px';
     },
 
 
   });
 
 </script>