Add aria-haspopup and aria-expanded attributes to dropdown plugin

1. Add aria-haspopup="true" and aria-expanded="false" to dropdown docs
2. Toggle aria-expanded between "true" and "false" when opening and closing dropdown

Satisfy item 1 from https://github.com/paypal/bootstrap-accessibility-plugin#dropdown. See #13553.

docs/_includes/js/dropdowns.html

@@ -20,7 +20,7 @@
         <div class="collapse navbar-collapse bs-example-js-navbar-collapse">
           <ul class="nav navbar-nav">
             <li class="dropdown">
-              <a id="drop1" href="#" role="button" class="dropdown-toggle" data-toggle="dropdown">Dropdown <span class="caret"></span></a>
+              <a id="drop1" href="#" role="button" class="dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
               <ul class="dropdown-menu" role="menu" aria-labelledby="drop1">
                 <li role="presentation"><a role="menuitem" tabindex="-1" href="http://twitter.com/fat">Action</a></li>
                 <li role="presentation"><a role="menuitem" tabindex="-1" href="http://twitter.com/fat">Another action</a></li>
@@ -30,7 +30,7 @@
               </ul>
             </li>
             <li class="dropdown">
-              <a href="#" id="drop2" role="button" class="dropdown-toggle" data-toggle="dropdown">Dropdown 2 <span class="caret"></span></a>
+              <a href="#" id="drop2" role="button" class="dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Dropdown 2 <span class="caret"></span></a>
               <ul class="dropdown-menu" role="menu" aria-labelledby="drop2">
                 <li role="presentation"><a role="menuitem" tabindex="-1" href="http://twitter.com/fat">Action</a></li>
                 <li role="presentation"><a role="menuitem" tabindex="-1" href="http://twitter.com/fat">Another action</a></li>
@@ -42,7 +42,7 @@
           </ul>
           <ul class="nav navbar-nav navbar-right">
             <li id="fat-menu" class="dropdown">
-              <a href="#" id="drop3" role="button" class="dropdown-toggle" data-toggle="dropdown">Dropdown 3 <span class="caret"></span></a>
+              <a href="#" id="drop3" role="button" class="dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Dropdown 3 <span class="caret"></span></a>
               <ul class="dropdown-menu" role="menu" aria-labelledby="drop3">
                 <li role="presentation"><a role="menuitem" tabindex="-1" href="http://twitter.com/fat">Action</a></li>
                 <li role="presentation"><a role="menuitem" tabindex="-1" href="http://twitter.com/fat">Another action</a></li>
@@ -62,7 +62,7 @@
     <ul class="nav nav-pills">
       <li class="active"><a href="#">Regular link</a></li>
       <li class="dropdown">
-        <a id="drop4" role="button" data-toggle="dropdown" href="#">Dropdown <span class="caret"></span></a>
+        <a id="drop4" role="button" data-toggle="dropdown" href="#" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
         <ul id="menu1" class="dropdown-menu" role="menu" aria-labelledby="drop4">
           <li role="presentation"><a role="menuitem" tabindex="-1" href="http://twitter.com/fat">Action</a></li>
           <li role="presentation"><a role="menuitem" tabindex="-1" href="http://twitter.com/fat">Another action</a></li>
@@ -72,7 +72,7 @@
         </ul>
       </li>
       <li class="dropdown">
-        <a id="drop5" role="button" data-toggle="dropdown" href="#">Dropdown 2 <span class="caret"></span></a>
+        <a id="drop5" role="button" data-toggle="dropdown" href="#" aria-haspopup="true" aria-expanded="false">Dropdown 2 <span class="caret"></span></a>
         <ul id="menu2" class="dropdown-menu" role="menu" aria-labelledby="drop5">
           <li role="presentation"><a role="menuitem" tabindex="-1" href="http://twitter.com/fat">Action</a></li>
           <li role="presentation"><a role="menuitem" tabindex="-1" href="http://twitter.com/fat">Another action</a></li>
@@ -82,7 +82,7 @@
         </ul>
       </li>
       <li class="dropdown">
-        <a id="drop6" role="button" data-toggle="dropdown" href="#">Dropdown 3 <span class="caret"></span></a>
+        <a id="drop6" role="button" data-toggle="dropdown" href="#" aria-haspopup="true" aria-expanded="false">Dropdown 3 <span class="caret"></span></a>
         <ul id="menu3" class="dropdown-menu" role="menu" aria-labelledby="drop6">
           <li role="presentation"><a role="menuitem" tabindex="-1" href="http://twitter.com/fat">Action</a></li>
           <li role="presentation"><a role="menuitem" tabindex="-1" href="http://twitter.com/fat">Another action</a></li>
@@ -102,7 +102,7 @@
   <p>Add <code>data-toggle="dropdown"</code> to a link or button to toggle a dropdown.</p>
 {% highlight html %}
 <div class="dropdown">
-  <a data-toggle="dropdown" href="#">Dropdown trigger</a>
+  <a data-toggle="dropdown" href="#" aria-haspopup="true" aria-expanded="false">Dropdown trigger</a>
   <ul class="dropdown-menu" role="menu" aria-labelledby="dLabel">
     ...
   </ul>
@@ -111,7 +111,7 @@
   <p>To keep URLs intact, use the <code>data-target</code> attribute instead of <code>href="#"</code>.</p>
 {% highlight html %}
 <div class="dropdown">
-  <a id="dLabel" role="button" data-toggle="dropdown" data-target="#" href="/page.html">
+  <a id="dLabel" role="button" data-toggle="dropdown" data-target="#" href="/page.html" aria-haspopup="true" aria-expanded="false">
     Dropdown <span class="caret"></span>
   </a>
 

js/dropdown.js

@@ -42,7 +42,9 @@
 
       if (e.isDefaultPrevented()) return
 
-      $this.trigger('focus')
+      $this
+        .trigger('focus')
+        .attr('aria-expanded', 'true')
 
       $parent
         .toggleClass('open')
@@ -88,11 +90,13 @@
     if (e && e.which === 3) return
     $(backdrop).remove()
     $(toggle).each(function () {
-      var $parent = getParent($(this))
+      var $this = $(this)
+      var $parent = getParent($this)
       var relatedTarget = { relatedTarget: this }
       if (!$parent.hasClass('open')) return
       $parent.trigger(e = $.Event('hide.bs.dropdown', relatedTarget))
       if (e.isDefaultPrevented()) return
+      $this.attr('aria-expanded', 'false')
       $parent.removeClass('open').trigger('hidden.bs.dropdown', relatedTarget)
     })
   }