Type extension support in polyfill.

tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate

 // Copyright (c) 2012, 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.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
@@ skipping 149 matching lines @@
   Element get pointerLockElement =>
       _webkitPointerLockElement;
 
   @DomName('Document.webkitVisibilityState')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
   @Experimental()
   String get visibilityState => _webkitVisibilityState;
 
   @Experimental
+  /**
+   * Register a custom subclass of Element to be instantiatable by the DOM.
+   *
+   * This is necessary to allow the construction of any custom elements.
+   *
+   * The class being registered must either subclass HtmlElement or SvgElement.
+   * If they subclass these directly then they can be used as:
+   *
+   *     class FooElement extends HtmlElement{
+   *        void created() {
+   *          print('FooElement created!');
+   *        }
+   *     }
+   *
+   *     main() {
+   *       document.register('x-foo', FooElement);
+   *       var myFoo = new Element.tag('x-foo');
+   *       // prints 'FooElement created!' to the console.
+   *     }
+   *
+   * The custom element can also be instantiated via HTML using the syntax
+   * `<x-foo></x-foo>`
+   *
+   * Other elements can be subclassed as well:
+   *
+   *     class BarElement extends InputElement{
+   *        void created() {
+   *          print('BarElement created!');
+   *        }
+   *     }
+   *
+   *     main() {
+   *       document.register('x-bar', BarElement);
+   *       var myBar = new Element.tag('input', 'x-bar');
+   *       // prints 'BarElement created!' to the console.
+   *     }
+   *
+   * This custom element can also be instantiated via HTML using the syntax
+   * `<input is="x-bar"></input>`
+   *
+   * The [nativeTagName] parameter is needed by platforms without native support
+   * when subclassing a native type other than:
+   *
+   * * HtmlElement
+   * * SvgElement
+   * * AnchorElement
+   * * AudioElement
+   * * ButtonElement
+   * * CanvasElement
+   * * DivElement
+   * * ImageElement
+   * * InputElement
+   * * LIElement
+   * * LabelElement
+   * * MenuElement
+   * * MeterElement
+   * * OListElement
+   * * OptionElement
+   * * OutputElement
+   * * ParagraphElement
+   * * PreElement
+   * * ProgressElement
+   * * SelectElement
+   * * SpanElement
+   * * UListElement
+   * * VideoElement
+   */
 $if DART2JS
-  void register(String tag, Type customElementClass) {
-    _registerCustomElement(JS('', 'window'), this, tag, customElementClass);
+  void register(String tag, Type customElementClass, {String nativeTagName}) {
+    _registerCustomElement(JS('', 'window'), this, tag, customElementClass,
+        nativeTagName);
   }
 $else
-  void register(String tag, Type custom) {
-    _Utils.register(tag, custom);
+  void register(String tag, Type customElementClass, {String nativeTagName}) {
+    _Utils.register(tag, customElementClass);
   }
 $endif
 
 $if DART2JS
   @Creates('Null')  // Set from Dart code; does not instantiate a native type.
 $endif
   // Note: used to polyfill <template>
   Document _templateContentsOwner;
 }