Fix some warnings in script.

components/script/dom/htmlelement.rs

@@ -200,7 +200,7 @@ impl<'a> VirtualMethods for JSRef<'a, HTMLElement> {
                                         window.r().reflector().get_jsobject());
             let evtarget: JSRef<EventTarget> = EventTargetCast::from_ref(*self);
             evtarget.set_event_handler_uncompiled(cx, url, reflector,
-                                                  name.slice_from(2),
+                                                  &name[2..],
                                                   attr.value().as_slice().to_owned());
         }
     }