Sample test for custom elements.

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

Index: tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
index c64f11a30730523044b3d42b4c424b3a8aa0ceef..f2777b6f3ac702e97fa69b03e42f56327a17558e 100644
--- a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
@@ -16,6 +16,17 @@ $!MEMBERS
     document.$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);