Which file I am missing to include

[Mandeep]

I am trying to use ExtJS without cmd and has following code, but it keeps failing with error
Line: 16481Error: 'Event' is undefined

Could somebody please tell if I am missing any files that I should be including.

<link href="/chart/theme-classic-all-debug.css">
<script src="/chart/ext-all-debug.js"></script>
<script src="/chart/theme-classic-debug.js"></script>


<script>
Ext.onReady(function(){
Ext.Msg.alert("", 'Ready to go!');
});
</script>

[Mandeep]

Any help on this will be appreciated.