fix(dropdown): handle keyboard-nav correctly

- fix bug in directive whereby list item selector is not being initiated on
  correct DOM element.
- fix bug in directive whereby invalid `keyCode` property of jQuery Event
  object is being referenced instead of `which`.
- update functional test to trigger keydown event on correct DOM element
  because otherwise the keydown handler in the directive is not getting
  executed.
- remove invalid functional test because it is not possible to trigger a
  keydown event on a hidden DOM element through normal UI interaction.

Closes #4110
Fixes #4091

src/dropdown/dropdown.js

@@ -278,9 +278,9 @@ angular.module('ui.bootstrap.dropdown', ['ui.bootstrap.position'])
           e.preventDefault();
           e.stopPropagation();
 
-          var elems = angular.element(element).find('a');
+          var elems = dropdownCtrl.dropdownMenu.find('a');
 
-          switch (e.keyCode) {
+          switch (e.which) {
             case (40): { // Down
               if ( !angular.isNumber(dropdownCtrl.selectedOption)) {
                 dropdownCtrl.selectedOption = 0;

src/dropdown/test/dropdown.spec.js

@@ -646,25 +646,17 @@ describe('dropdownToggle', function() {
     it('should focus first list element when down arrow pressed', function() {
       clickDropdownToggle();
 
-      triggerKeyDown($document, 40);
+      triggerKeyDown(element, 40);
 
       expect(element.hasClass(dropdownConfig.openClass)).toBe(true);
       var focusEl = $document.find('ul').eq(0).find('a');
       expect(isFocused(focusEl)).toBe(true);
     });
 
-    it('should not focus first list element when down arrow pressed if closed', function() {
-      triggerKeyDown($document, 40);
-
-      expect(element.hasClass(dropdownConfig.openClass)).toBe(false);
-      var focusEl = $document.find('ul').eq(0).find('a');
-      expect(isFocused(focusEl)).toBe(false);
-    });
-
     it('should focus second list element when down arrow pressed twice', function() {
       clickDropdownToggle();
-      triggerKeyDown($document, 40);
-      triggerKeyDown($document, 40);
+      triggerKeyDown(element, 40);
+      triggerKeyDown(element, 40);
 
       expect(element.hasClass(dropdownConfig.openClass)).toBe(true);
       var elem1 = $document.find('ul');