update elements and fix some bugs

lib/paper_card.dart

 // DO NOT EDIT: auto-generated with `pub run custom_element_apigen:update`
 
 /// Dart API for the polymer element `paper_card`.
 @HtmlImport('paper_card_nodart.html')
 library polymer_elements.lib.src.paper_card.paper_card;
 
 import 'dart:html';
 import 'dart:js' show JsArray, JsObject;
 import 'package:web_components/web_components.dart';
 import 'package:polymer_interop/polymer_interop.dart';
 import 'paper_material.dart';
 import 'iron_flex_layout.dart';
 
 /// Material Design: <a href="http://www.google.com/design/spec/components/cards.html">Cards</a>
 ///
 /// `paper-card` is a container with a drop shadow.
 ///
 /// Example:
 ///
 ///     <paper-card heading="Card Title">
 ///       <div class="card-content">Some content</div>
 ///       <div class="card-actions">
 ///         <paper-button>Some action</paper-button>
 ///       </div>
 ///     </paper-card>
 ///
+/// Example - top card image:
+///
+///     <paper-card heading="Card Title" image="/path/to/image.png">
+///       ...
+///     </paper-card>
+///
 /// ### Accessibility
 ///
 /// By default, the `aria-label` will be set to the value of the `heading` attribute.
 ///
 /// ### Styling
 ///
 /// The following custom properties and mixins are available for styling:
 ///
 /// Custom property | Description | Default
 /// ----------------|-------------|----------
@@ skipping 20 matching lines @@
   set elevation(num value) { jsElement[r'elevation'] = value; }
 
   /// The title of the card.
   String get heading => jsElement[r'heading'];
   set heading(String value) { jsElement[r'heading'] = value; }
 
   /// The url of the title image of the card.
   String get image => jsElement[r'image'];
   set image(String value) { jsElement[r'image'] = value; }
 }