catching Window closing event in pyjd

This is a normal way to catch a window closing event. It worked in web-browser versions, but not in pyjd.
    
    from pyjamas import Window
    class CloseHandler(object):
        def __init__(self):
            Window.addWindowCloseListener(self)
        def onWindowClosed(self):
            Window.alert('in onWindowClosed()')
        def onWindowClosing(self):
            Window.alert('in onWindowClosing()')

I found that Browser.on_unload_callback() in mshtml.py catches the window close event, and it did nothing. I modified Browser() class to add event handlers, and pyjd to return the browser object created.  This is how to start pyjd to add a windnow close handler:

    import pyjd
    from pyjamas import Window
    browser=pyjd.setup(url)
    browser.addCloseHandler(Window.onClosing)
    
Now,  the added handler will catch the event and call CloseHandler.onWindowClosing().  I don't know how to catch onWindowClosed, Browser.on_before_unload_callback() doesn't exist. 

The current GitHub version modified after  pip installation doesn't have pyjd installation, so I tested my code in the release version pyjs-0.8.1a.