Make html metadata follow the spec.

tools/dom/templates/html/impl/impl_HTMLTemplateElement.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;
 
 
 /**
@@ skipping 21 matching lines @@
  *
  *     ...
  *
  *     TemplateElement.syntax['MySyntax'] = new MySyntax();
  *
  * See <https://github.com/polymer-project/mdv/blob/master/docs/syntax.md> for
  * more information about Custom Syntax.
  */
 // TODO(jmesserly): if this is just one method, a function type would make it
 // more Dart-friendly.
-@Experimental
+@Experimental()
 abstract class CustomBindingSyntax {
   /**
    * This syntax method allows for a custom interpretation of the contents of
    * mustaches (`{{` ... `}}`).
    *
    * When a template is inserting an instance, it will invoke this method for
    * each mustache which is encountered. The function is invoked with four
    * arguments:
    *
    * - [model]: The data context for which this instance is being created.
@@ skipping 39 matching lines @@
    * This syntax method allows a syntax to provide an alterate expansion of
    * the [template] contents. When the template wants to create an instance,
    * it will call this method with the template element.
    *
    * By default this will call `template.createInstance()`.
    */
   getInstanceFragment(Element template) => template.createInstance();
 }
 
 
-@Experimental
+@Experimental()
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
 
   // For real TemplateElement use the actual DOM .content field instead of
   // our polyfilled expando.
-  @Experimental
+  @Experimental()
   DocumentFragment get content => $dom_content;
 
 
   /**
    * The MDV package, if available.
    *
    * This can be used to initialize MDV support via:
    *
    *     import 'dart:html';
    *     import 'package:mdv/mdv.dart' as mdv;
@@ skipping 10 matching lines @@
   /**
    * Ensures proper API and content model for template elements.
    *
    * [instanceRef] can be used to set the [Element.ref] property of [template],
    * and use the ref's content will be used as source when createInstance() is
    * invoked.
    *
    * Returns true if this template was just decorated, or false if it was
    * already decorated.
    */
-  @Experimental
+  @Experimental()
   static bool decorate(Element template, [Element instanceRef]) {
     // == true check because it starts as a null field.
     if (template._templateIsDecorated == true) return false;
 
     _injectStylesheet();
 
     var templateElement = template;
     var isNative = templateElement is TemplateElement;
     var bootstrapContents = isNative;
     var liftContents = !isNative;
@@ skipping 97 matching lines @@
   }
 
   /**
    * This used to decorate recursively all templates from a given node.
    *
    * By default [decorate] will be called on templates lazily when certain
    * properties such as [model] are accessed, but it can be run eagerly to
    * decorate an entire tree recursively.
    */
   // TODO(rafaelw): Review whether this is the right public API.
-  @Experimental
+  @Experimental()
   static void bootstrap(Node content) {
     void _bootstrap(template) {
       if (!TemplateElement.decorate(template)) {
         bootstrap(template.content);
       }
     }
 
     // Need to do this first as the contents may get lifted if |node| is
     // template.
     // TODO(jmesserly): content is DocumentFragment or Element
@@ skipping 30 matching lines @@
 option[template] {
   display: none;
 }''';
     document.head.append(style);
   }
 
   /**
    * A mapping of names to Custom Syntax objects. See [CustomBindingSyntax] for
    * more information.
    */
-  @Experimental
+  @Experimental()
   static Map<String, CustomBindingSyntax> syntax = {};
 }