Make html metadata follow the spec.

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 51 matching lines @@
    *     context.fillStyle = 'red';
    *     context.fillRect(0, 0, 100, 100);
    *
    * See also:
    *
    * * [supportsCssCanvasContext]
    * * [CanvasElement.getContext]
    */
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
-  @Experimental
+  @Experimental()
   @DomName('Document.getCSSCanvasContext')
   CanvasRenderingContext getCssCanvasContext(String contextId, String name,
       int width, int height) {
     return $dom_getCssCanvasContext(contextId, name, width, height);
   }
 
   @DomName('Document.head')
   HeadElement get head => $dom_head;
 
   @DomName('Document.lastModified')
@@ skipping 18 matching lines @@
   String get title => $dom_title;
 
   @DomName('Document.title')
   void set title(String value) {
     $dom_title = value;
   }
 
   @DomName('Document.webkitCancelFullScreen')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
-  @Experimental
+  @Experimental()
   void cancelFullScreen() {
     $dom_webkitCancelFullScreen();
   }
 
   @DomName('Document.webkitExitFullscreen')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
-  @Experimental
+  @Experimental()
   void exitFullscreen() {
     $dom_webkitExitFullscreen();
   }
 
   @DomName('Document.webkitExitPointerLock')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
-  @Experimental
+  @Experimental()
   void exitPointerLock() {
     $dom_webkitExitPointerLock();
   }
 
   @DomName('Document.webkitFullscreenElement')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
-  @Experimental
+  @Experimental()
   Element get fullscreenElement => $dom_webkitFullscreenElement;
 
   @DomName('Document.webkitFullscreenEnabled')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
-  @Experimental
+  @Experimental()
   bool get fullscreenEnabled => $dom_webkitFullscreenEnabled;
 
   @DomName('Document.webkitHidden')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
-  @Experimental
+  @Experimental()
   bool get hidden => $dom_webkitHidden;
 
   @DomName('Document.webkitIsFullScreen')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
-  @Experimental
+  @Experimental()
   bool get isFullScreen => $dom_webkitIsFullScreen;
 
   @DomName('Document.webkitPointerLockElement')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
-  @Experimental
+  @Experimental()
   Element get pointerLockElement =>
       $dom_webkitPointerLockElement;
 
   @DomName('Document.webkitVisibilityState')
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.SAFARI)
-  @Experimental
+  @Experimental()
   String get visibilityState => $dom_webkitVisibilityState;
 
 
 $if DART2JS
   @Creates('Null')  // Set from Dart code; does not instantiate a native type.
 $endif
   // Note: used to polyfill <template>
   Document _templateContentsOwner;
 }