As far as I know, the support for library://ns.adobe.com/flex/halo namespace has been dropped, and now we have to use library://ns.adobe.com/flex/mx instead (reference). Can someone provide if there's any difference between the two namespaces?
I am just starting to learn Flex and this change make me confused. For example, if I have an <mx:Tree> tag in my mxml document, the compiler complains that <mx:Tree> could not be resolved to a component implementation. But if I change my mx namespace to use the old one instead (halo), it successfully compiled without error.
Updated: By the way, I use Flex SDK command line compiler in Windows.
mxmlc --version
Version 4.0.0 build 10485
Adobe changed the "halo" namespace in order to more clearly differentiate it from the spark namespace. All you need to do is find/replace the old URI with the new one. The existing halo components will continue to work as is just with a new namespace.