update elements and fix some bugs

test/paper_dropdown_menu_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 @TestOn('browser')
 library polymer_elements.test.paper_dropdown_menu_test;
 
 import 'package:polymer_elements/paper_dropdown_menu.dart';
 import 'package:polymer_elements/paper_menu.dart';
 import 'package:polymer_interop/polymer_interop.dart';
 import 'package:web_components/web_components.dart';
@@ skipping 57 matching lines @@
     group('when a value is preselected', () {
       setUp(() {
         dropdownMenu = fixture('PreselectedDropdownMenu');
       });
 
       test('the input area shows the correct selection', () {
         var secondItem = Polymer.dom(dropdownMenu).querySelectorAll('paper-item')[1];
         expect(dropdownMenu.selectedItem, secondItem);
       });
     });
+    
+    group('deselecting', () {
+      var menu;
+
+      setUp(() {
+        dropdownMenu = fixture('PreselectedDropdownMenu');
+        menu = Polymer.dom(dropdownMenu).querySelector('.dropdown-content');
+      });
+
+      test('an `iron-deselect` event clears the current selection', () {
+        menu.selected = null;
+        expect(dropdownMenu.selectedItem, null);
+      });
+    });
+
+    group('validation', () {
+      var menu;
+
+      test('a non required dropdown is valid regardless of its selection', () {
+        PaperDropdownMenu dropdownMenu = fixture('TrivialDropdownMenu');
+        menu = Polymer.dom(dropdownMenu).querySelector('.dropdown-content');
+
+        // no selection.
+        expect(dropdownMenu.validate(null), isTrue);
+        expect(dropdownMenu.invalid, isFalse);
+        expect(dropdownMenu.value, isNull);
+
+        // some selection.
+        menu.selected = 1;
+        expect(dropdownMenu.validate(null), isTrue);
+        expect(dropdownMenu.invalid, isFalse);
+        expect(dropdownMenu.value, 'Bar');
+      });
+
+      test('a required dropdown is invalid without a selection', () {
+        PaperDropdownMenu dropdownMenu = fixture('TrivialDropdownMenu');
+        dropdownMenu.required = true;
+
+        // no selection.
+        expect(dropdownMenu.validate(null), isFalse);
+        expect(dropdownMenu.invalid, isTrue);
+        expect(dropdownMenu.value, isNull);
+      });
+
+      test('a required dropdown is valid with a selection', () {
+        PaperDropdownMenu dropdownMenu = fixture('PreselectedDropdownMenu');
+        dropdownMenu.required = true;
+        
+        expect(dropdownMenu.validate(null), isTrue);
+        expect(dropdownMenu.invalid, isFalse);
+        expect(dropdownMenu.value, 'Bar');
+      });
+    });
   });
 }