update elements and fix some bugs

lib/iron_form.dart

 // DO NOT EDIT: auto-generated with `pub run custom_element_apigen:update`
 
 /// Dart API for the polymer element `iron_form`.
 @HtmlImport('iron_form_nodart.html')
 library polymer_elements.lib.src.iron_form.iron_form;
 
 import 'dart:html';
 import 'dart:js' show JsArray, JsObject;
 import 'package:web_components/web_components.dart';
 import 'package:polymer_interop/polymer_interop.dart';
@@ skipping 35 matching lines @@
   set contentType(String value) { jsElement[r'contentType'] = value; }
 
   /// By default, the form will display the browser's native validation
   /// UI (i.e. popup bubbles and invalid styles on invalid fields). You can
   /// manually disable this; however, if you do, note that you will have to
   /// manually style invalid *native* HTML fields yourself, as you are
   /// explicitly preventing the native form from doing so.
   bool get disableNativeValidationUi => jsElement[r'disableNativeValidationUi'];
   set disableNativeValidationUi(bool value) { jsElement[r'disableNativeValidationUi'] = value; }
 
+  /// HTTP request headers to send
+  ///
+  /// Note: setting a `Content-Type` header here will override the value
+  /// specified by the `contentType` property of this element.
+  get headers => jsElement[r'headers'];
+  set headers(value) { jsElement[r'headers'] = (value is Map || (value is Iterable && value is! JsArray)) ? new JsObject.jsify(value) : value;}
+
   /// Set the withCredentials flag when sending data.
   bool get withCredentials => jsElement[r'withCredentials'];
   set withCredentials(bool value) { jsElement[r'withCredentials'] = value; }
 
   /// Returns a json object containing name/value pairs for all the registered
   /// custom components and native elements of the form. If there are elements
   /// with duplicate names, then their values will get aggregated into an
   /// array of values.
   serialize() =>
       jsElement.callMethod('serialize', []);
 
   /// Called to submit the form.
   submit() =>
       jsElement.callMethod('submit', []);
 
   /// Validates all the required elements (custom and native) in the form.
   bool validate() =>
       jsElement.callMethod('validate', []);
 }