Fixing experimental annotation on HtmlDocument.register

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 141 matching lines @@
   @SupportedBrowser(SupportedBrowser.IE, '10')
   @Experimental()
 $if DART2JS
   String get visibilityState => JS('String',
     '(#.visibilityState || #.mozVisibilityState || #.msVisibilityState ||'
       '#.webkitVisibilityState)', this, this, this, this);
 $else
   String get visibilityState => _webkitVisibilityState;
 $endif
 
-  @Experimental
+  @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() {
@@ skipping 98 matching lines @@
 $endif
   }
 
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.FIREFOX)
   @SupportedBrowser(SupportedBrowser.IE, '10')
   @Experimental()
   Stream<Event> get onVisibilityChange =>
       visibilityChangeEvent.forTarget(this);
 }