Fix MouseEvent.movement to not use old webkit prefix

tools/dom/templates/html/dartium/impl_MouseEvent.darttemplate

Index: tools/dom/templates/html/dartium/impl_MouseEvent.darttemplate
diff --git a/tools/dom/templates/html/dartium/impl_MouseEvent.darttemplate b/tools/dom/templates/html/dartium/impl_MouseEvent.darttemplate
index c4dc511110dd8b36191013dc7118f59382ae7a5e..ca70cf5027f8290030f874362f03dcc8e27caaa5 100644
--- a/tools/dom/templates/html/dartium/impl_MouseEvent.darttemplate
+++ b/tools/dom/templates/html/dartium/impl_MouseEvent.darttemplate
@@ -48,9 +48,9 @@ $!MEMBERS
   @DomName('MouseEvent.movementX')
   @DomName('MouseEvent.movementY')
   @SupportedBrowser(SupportedBrowser.CHROME)
-  @SupportedBrowser(SupportedBrowser.SAFARI)
+  @SupportedBrowser(SupportedBrowser.FIREFOX)
   @Experimental()
-  Point get movement => new Point(_webkitMovementX, _webkitMovementY);
+  Point get movement => new Point(movementX, movementY);
 
   /**
    * The coordinates of the mouse pointer in target node coordinates.