Bug 120544 [build path] Editing external source folder does not persist
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/CPListElement.java
* Entries without resource are either non existing or a variable entry
	 * External jars do not have a resource
	 */
	public IResource getResource() {
		return fResource;
	}
	
	public CPListElementAttribute setAttribute(String key, Object value) {
		CPListElementAttribute attribute= findAttributeElement(key);
		if (attribute == null) {
			return null;
		}
		if (key.equals(EXCLUSION) || key.equals(INCLUSION)) {
			Assert.isTrue(value != null || fEntryKind != IClasspathEntry.CPE_SOURCE);
		}
		
		if (key.equals(ACCESSRULES)) {
			Assert.isTrue(value != null || fEntryKind == IClasspathEntry.CPE_SOURCE);
		}
		if (key.equals(COMBINE_ACCESSRULES)) {
			Assert.isTrue(value instanceof Boolean);
		}
		
		attribute.setValue(value);
		attributeChanged(key);
		return attribute;
	}
	
	public boolean addToExclusions(IPath path) {
		String key= CPListElement.EXCLUSION;