B can not be found in source packages. Check the inheritance chain from your module; it may not be inheriting a required module or a module may not be adding its source path entries properly.However these errors affect nothing. You can still run as / debug as web application in development mode, or GWT compile the whole project successfully.
I'm using Eclipse Helios 3.6.1 (Service Release 1) in Ubuntu with
- Google Plugin for Eclipse
- GWT Designer
After investigation, I found it should be a bug of GWT Designer and reported it to GWT team.
If you really mind the "errors" and need to use GWT Designer, I think you know how to deal with it. In case you want to get rid of GWT Designer, go to Help - About Eclipse - Installation Details - Installed Software, and uninstall all the items with Id starting with com.instantiations.
18/11/2010 Update: It's a bug of Enhanced Compilation and got fixed in GWT Designer BETA.