The Flex 4 ‘halo’ namespace is now the ‘mx’ namespace

With Flex SDK SVN trunk revision 12635 the Flex 4 SDK no longer supports the “library://ns.adobe.com/flex/halo” component namespace. MX components should be referenced using the “library://ns.adobe.com/flex/mx” namespace.

For example:
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
    xmlns:s="library://ns.adobe.com/flex/spark"
    xmlns:mx="library://ns.adobe.com/flex/mx">

For more information see http://bugs.adobe.com/jira/browse/SDK-24568 and http://forums.adobe.com/message/2445911.