Make html metadata follow the spec.

sdk/lib/html/html_common/metadata.dart

 // 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.
 
 library metadata;
 
 /**
  * An annotation used to mark a feature as only being supported by a subset
  * of the browsers that Dart supports by default.
  *
@@ skipping 20 matching lines @@
 /**
  * An annotation used to mark an API as being experimental.
  *
  * An API is considered to be experimental if it is still going through the
  * process of stabilizing and is subject to change or removal.
  *
  * See also:
  *
  * * [W3C recommendation](http://en.wikipedia.org/wiki/W3C_recommendation)
  */
-class Experimental {}
+class Experimental {
+  const Experimental();
+}
 
 
 /**
  * Annotation that specifies that a member is editable through generate files.
  *
  * This is used for API generation.
  *
  * [name] should be formatted as `interface.member`.
  */
 class DomName {
   final String name;
   const DomName(this.name);
 }
 
 
 /**
  * Metadata that specifies that that member is editable through generated
  * files.
  */
-class DocsEditable {}
+class DocsEditable {
+  const DocsEditable();
+}
 
 
 /**
  * Annotation that indicates that an API is not expected to change but has
  * not undergone enough testing to be considered stable.
  */
-class Unstable {}
+class Unstable {
+  const Unstable();
+}