To have an actionscript based project you need to create an actionscript project then you can declare which as file is your default for the application build.
David's right--in Flash Builder you need to create a new ActionScript project to get that workflow to work.
You could also create an MXML file based on your AS file and get that to run in any Flex project in Flash Builder. See http://frishy.blogspot.com/2008/07/creating-frameworkless-mxml-file.html
Thanks for the responses.
It's lame that you have to create a new project to convert your application file from .mxml to .as. What is even worse is that in FlashBuilder 4 if you create an ActionScript project you can't use the Spark components. I've opened a bug for this problem:
The ActionScript project workflow is typically used for non-Flex projects. That's why you see that many SWCs are not present (like spark.swc) in an ActionScript project. This is by design. You can manually re-add these SWCs in the project properties Flex Builder Path > Library path under the Flex 4.0 node.
Based on your posts though, this doesn't sound like the desired setup. You want to use the Spark controls in the Flex 4.0 framework but have an ActionScript class as the main application. The framework-less recomendation made earlier doesn't seem right either.
What might work better for you is the code behind technique found here http://www.adobe.com/devnet/flex/quickstart/building_components_using_code_behind/. You can google "code behind" and find a lot of discussion about this.
Essentially, for what you've described so far, you'll want to: create a Flex project, subclass SparkApplication (e.g. MyApplication.as), then have your main application file subclass that (e.g. the root element would be <local:MyApplication...).
Jason San Jose
Quality Engineer, Flash Builder
There's no point in using a mxml application file with <local:MyApplication.../>. MyApplication.as would serve no purpose then. I can use a mxml application file that just launches the ActionScript, but that's kind of silly.
Why is it necessary with Flex projects to have the Application file be mxml?