Sample test for custom elements.

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 $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
   @DomName('Document.body')
   BodyElement get body => document.$dom_body;
 
   @DomName('Document.body')
   void set body(BodyElement value) {
     document.$dom_body = value;
   }
 
+  /**
+   * Registers a custom Element subclass as an available HTML tag.
+   *
+   * Not yet implemented.
+   */
+  @Experimental
+  void register(String tagName, Type elementClass) {
+    // TODO: tagName validation
+    throw new Exception('Not yet implemented');
+  }
+
   @DomName('Document.caretRangeFromPoint')
   Range caretRangeFromPoint(int x, int y) {
     return document.$dom_caretRangeFromPoint(x, y);
   }
 
   @DomName('Document.elementFromPoint')
   Element elementFromPoint(int x, int y) {
     return document.$dom_elementFromPoint(x, y);
   }
 
@@ skipping 97 matching lines @@
   @DomName('Document.webkitIsFullScreen')
   bool get webkitIsFullScreen => document.$dom_webkitIsFullScreen;
 
   @DomName('Document.webkitPointerLockElement')
   Element get webkitPointerLockElement =>
       document.$dom_webkitPointerLockElement;
 
   @DomName('Document.webkitVisibilityState')
   String get webkitVisibilityState => document.$dom_webkitVisibilityState;
 }