Mark jQuery methods as experimental.

tools/dom/templates/html/impl/impl_Element.darttemplate

Index: tools/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Element.darttemplate b/tools/dom/templates/html/impl/impl_Element.darttemplate
index 4f4241ebeaa8a207b2112a6952499a82d33c616a..e3e769c7fe7faebe383fa6ba723524e9df11114d 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -1218,6 +1218,7 @@ $endif
    * This method is the Dart equivalent to jQuery's
    * [offset](http://api.jquery.com/offset/) method.
    */
+  @Experimental()
   Point get documentOffset => offsetTo(document.documentElement);
 
   /**
@@ -1231,6 +1232,7 @@ $endif
    * element is _not_ an offset parent or transitive offset parent to this
    * element, an [ArgumentError] is thrown.
    */
+  @Experimental()
   Point offsetTo(Element parent) {
     return Element._offsetToHelper(this, parent);
   }