KmlDataSource Problem

I was trying to add my KML data to the globe using this codes :

var kmlDataSource = new Cesium.KmlDataSource();

My KML data and the globe cannot show

My fire bug detects something like this :

TypeError: Cesium.KmlDataSource is not a constructor

Do I need to get external javascript file like KmlDataSource.js?
(I have read one topic in a google gropus forum that someone parsed his codes to the forum about KmlDataSource)
But I haven't found it in thirdparty folder or anywhere

I use Cesium ver 1.4

Can somebody help me here please...?

Thanks very much

KML support is not part of any official Cesium release yet but is being worked on in the KML branch. I plan on starting to post beta builds for people to play with soon (since so many people have been trying to build it from source). Expect a post to the forum as early as tomorrow.