var btn = document.getElementById('<%=btnDummy.ClientID%>');
This is something i have used previously and can confirm it works in IE. , also try moving the the script block to the end of the page, it might also have something to the version of IE you are using.
Or you can try this, this is something i used in Safari which wouldnt otherwise fire the click() event properly
var dispatch = document.createEvent("HTMLEvents")
dispatch.initEvent("click", true, true);
Hope it helps.