I've tried this with the default 4.0 SDK, and milestone build 13875, same results;
The Flash Builder, when set to a MX + Spark project, doesn't seem to find any number of classes it should find, for example, URLLoader, URLRequest, Event, etc. When I set the project to just MX, everything works fine. The classes don't even come up in autocomplete, manually putting in the import statements just results in more classes not found.
This happens in the most basic test case; start a project, in the default app form, put in an fx:script block and try to create a URLLoader var, you get an error.
I figure this is some kind of source path/library path bug or some such. What's the resolution?
Thanks for the info, note please that the build I indicate is not a nightly build, it is the most recent milestone build.