Flash Builder 4 is suggesting mx1 instead of mx! why?

I just bought and installed the Flash Builder 4 after having the Beta for a while. The same code is giving me compile-time errors and 开发者_Python百科suggests using mx1 instead of mx! If I make it mx1 the compile error goes away. Here is the top of my component:

<s:SkinnableContainer   xmlns:fx="http://ns.adobe.com/mxml/2009"
                        xmlns:s="library://ns.adobe.com/flex/spark"
                        xmlns:mx="library://ns.adobe.com/flex/halo"

I mentioned nothing about mx1.

I used:

xmlns:mx="library://ns.adobe.com/flex/mx"

instead of:

xmlns:mx="library://ns.adobe.com/flex/halo"

and it worked.