update elements and fix some bugs

lib/paper_dropdown_menu.dart

 // DO NOT EDIT: auto-generated with `pub run custom_element_apigen:update`
 
 /// Dart API for the polymer element `paper_dropdown_menu`.
 @HtmlImport('paper_dropdown_menu_nodart.html')
 library polymer_elements.lib.src.paper_dropdown_menu.paper_dropdown_menu;
 
 import 'dart:html';
 import 'dart:js' show JsArray, JsObject;
 import 'package:web_components/web_components.dart';
 import 'package:polymer_interop/polymer_interop.dart';
 import 'iron_control_state.dart';
 import 'iron_button_state.dart';
 import 'iron_a11y_keys_behavior.dart';
+import 'iron_form_element_behavior.dart';
+import 'iron_validatable_behavior.dart';
 import 'default_theme.dart';
 import 'paper_input.dart';
 import 'paper_menu_button.dart';
 import 'paper_ripple.dart';
 import 'iron_icons.dart';
 import 'iron_icon.dart';
 import 'iron_selectable.dart';
 
+/// Material design: [Dropdown menus](https://www.google.com/design/spec/components/buttons.html#buttons-dropdown-buttons)
+///
 /// `paper-dropdown-menu` is similar to a native browser select element.
 /// `paper-dropdown-menu` works with selectable content. The currently selected
 /// item is displayed in the control. If no item is selected, the `label` is
 /// displayed instead.
 ///
 /// The child element with the class `dropdown-content` will be used as the dropdown
 /// menu. It could be a `paper-menu` or element that triggers `iron-select` when
 /// selecting its children.
 ///
 /// Example:
 ///
 ///     <paper-dropdown-menu label="Your favourite pastry">
 ///       <paper-menu class="dropdown-content">
 ///         <paper-item>Croissant</paper-item>
 ///         <paper-item>Donut</paper-item>
 ///         <paper-item>Financier</paper-item>
 ///         <paper-item>Madeleine</paper-item>
 ///       </paper-menu>
 ///     </paper-dropdown-menu>
 ///
 /// This example renders a dropdown menu with 4 options.
 ///
+/// Similarly to using `iron-select`, `iron-deselect` events will cause the
+/// current selection of the `paper-dropdown-menu` to be cleared.
+///
 /// ### Styling
 ///
 /// The following custom properties and mixins are also available for styling:
 ///
 /// Custom property | Description | Default
 /// ----------------|-------------|----------
 /// `--paper-dropdown-menu` | A mixin that is applied to the element host | `{}`
 /// `--paper-dropdown-menu-disabled` | A mixin that is applied to the element host when disabled | `{}`
 /// `--paper-dropdown-menu-ripple` | A mixin that is applied to the internal ripple | `{}`
 /// `--paper-dropdown-menu-button` | A mixin that is applied to the internal menu button | `{}`
 /// `--paper-dropdown-menu-input` | A mixin that is applied to the internal paper input | `{}`
 /// `--paper-dropdown-menu-icon` | A mixin that is applied to the internal icon | `{}`
 ///
 /// You can also use any of the `paper-input-container` and `paper-menu-button`
 /// style mixins and custom properties to style the internal input and menu button
 /// respectively.
 @CustomElementProxy('paper-dropdown-menu')
-class PaperDropdownMenu extends HtmlElement with CustomElementProxyMixin, PolymerBase, IronControlState, IronA11yKeysBehavior, IronButtonState {
+class PaperDropdownMenu extends HtmlElement with CustomElementProxyMixin, PolymerBase, IronControlState, IronA11yKeysBehavior, IronButtonState, IronFormElementBehavior, IronValidatableBehavior {
   PaperDropdownMenu.created() : super.created();
   factory PaperDropdownMenu() => new Element.tag('paper-dropdown-menu');
 
   /// Set to true to always float the label. Bind this to the
   /// `<paper-input-container>`'s `alwaysFloatLabel` property.
   bool get alwaysFloatLabel => jsElement[r'alwaysFloatLabel'];
   set alwaysFloatLabel(bool value) { jsElement[r'alwaysFloatLabel'] = value; }
 
   /// The content element that is contained by the dropdown menu, if any.
   get contentElement => jsElement[r'contentElement'];
@@ skipping 28 matching lines @@
   /// `iron-select` event with the selected `item` in the `detail`.
   get selectedItem => jsElement[r'selectedItem'];
   set selectedItem(value) { jsElement[r'selectedItem'] = (value is Map || (value is Iterable && value is! JsArray)) ? new JsObject.jsify(value) : value;}
 
   /// The derived "label" of the currently selected item. This value
   /// is the `label` property on the selected item if set, or else the
   /// trimmed text content of the selected item.
   String get selectedItemLabel => jsElement[r'selectedItemLabel'];
   set selectedItemLabel(String value) { jsElement[r'selectedItemLabel'] = value; }
 
+  /// The value for this element that will be used when submitting in
+  /// a form. It is read only, and will always have the same value
+  /// as `selectedItemLabel`.
+  String get value => jsElement[r'value'];
+  set value(String value) { jsElement[r'value'] = value; }
+
   /// Hide the dropdown content.
   close() =>
       jsElement.callMethod('close', []);
 
   /// Show the dropdown content.
   open() =>
       jsElement.callMethod('open', []);
 }