Make html metadata follow the spec.

tools/dom/templates/html/dartium/impl_MouseEvent.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 {
   factory $CLASSNAME(String type,
@@ skipping 31 matching lines @@
   int get screenY => screen.y;
 
   @DomName('MouseEvent.clientX')
   @DomName('MouseEvent.clientY')
   Point get client => new Point($dom_clientX, $dom_clientY);
 
   @DomName('MouseEvent.movementX')
   @DomName('MouseEvent.movementY')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
-  @Experimental
+  @Experimental()
   Point get movement => new Point($dom_webkitMovementX, $dom_webkitMovementY);
 
   /**
    * The coordinates of the mouse pointer in target node coordinates.
    *
    * This value may vary between platforms if the target node moves
    * after the event has fired or if the element has CSS transforms affecting
    * it.
    */
   Point get offset => new Point($dom_offsetX, $dom_offsetY);
 
   @DomName('MouseEvent.screenX')
   @DomName('MouseEvent.screenY')
   Point get screen => new Point($dom_screenX, $dom_screenY);
 }