update elements and fix some bugs

lib/src/iron-collapse/iron-collapse.html

Index: lib/src/iron-collapse/iron-collapse.html
diff --git a/lib/src/iron-collapse/iron-collapse.html b/lib/src/iron-collapse/iron-collapse.html
index 96e475faf18fc610ae705977123c23abe02907f6..013660e5ed5e76adf193ef2abce481c53c457a07 100644
--- a/lib/src/iron-collapse/iron-collapse.html
+++ b/lib/src/iron-collapse/iron-collapse.html
@@ -162,6 +162,9 @@ and instead put a div inside and style that.
 
     _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();
@@ -169,16 +172,18 @@ and instead put a div inside and style that.
         // 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() {