Bug 13799 JAR exporter exports class files from output folder
There was a problem with the output folder not being detected as such.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/jarpackager/JarFileExportOperation.java
else
			return true; 
	}	
	private void buildProjects(IProgressMonitor progressMonitor) {
		Set builtProjects= new HashSet(10);
		Object[] elements= fJarPackage.getElements();
		for (int i= 0; i < elements.length; i++) {
			IProject project= null;
			Object element= elements[i];
			if (element instanceof IResource)
				project= ((IResource)element).getProject();
			else if (element instanceof IJavaElement)
				project= ((IJavaElement)element).getJavaProject().getProject();
			if (project != null && !builtProjects.contains(project)) {
				try {
					project.build(IncrementalProjectBuilder.INCREMENTAL_BUILD, progressMonitor);
				} catch (CoreException ex) {
					String message= JarPackagerMessages.getFormattedString("JarFileExportOperation.errorDuringProjectBuild", project.getFullPath()); 
					addError(message, ex);
				} finally {
					
					builtProjects.add(project);
				}
			}
		}
	}
	/**
	 * Tells whether the given resource (or its children) have compile errors.
	 * The method acts on the current build state and does not recompile.
	 *