Bug 123786 Following bug 109980, the compliance 6.0 should be added to the compiler's compliance list.
If the JRE/JDK used is a 1.6/6.0 VM, then the compliance 6.0 should be available. Compliance 1.6 implies source level 1.6 and target level 1.6.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/BuildPathSupport.java
* @param containerPath The path of the entry's parent container or <code>null</code> if the entry is not in a container
	 * @param monitor The progress monitor to use
	 * @throws CoreException
	 */
	public static void modifyClasspathEntry(Shell shell, IClasspathEntry newEntry, IJavaProject jproject, IPath containerPath, IProgressMonitor monitor) throws CoreException {
		modifyClasspathEntry(shell, newEntry, null, jproject, containerPath, monitor);
	}
	private static void updateContainerClasspath(IJavaProject jproject, IPath containerPath, IClasspathEntry newEntry, String[] changedAttributes, IProgressMonitor monitor) throws CoreException {
		IClasspathContainer container= JavaCore.getClasspathContainer(containerPath, jproject);
		if (container == null) {
			throw new CoreException(new Status(IStatus.ERROR, JavaUI.ID_PLUGIN, IStatus.ERROR, "Container " + containerPath + " cannot be resolved", null));  
		}
		IClasspathEntry[] entries= container.getClasspathEntries();
		IClasspathEntry[] newEntries= new IClasspathEntry[entries.length];
		for (int i= 0; i < entries.length; i++) {
			IClasspathEntry curr= entries[i];
			if (curr.getEntryKind() == newEntry.getEntryKind() && curr.getPath().equals(newEntry.getPath())) {
				newEntries[i]= getUpdatedEntry(curr, newEntry, changedAttributes, jproject);
			} else {
				newEntries[i]= curr;
			}
		}
		requestContainerUpdate(jproject, container, newEntries);
		monitor.worked(1);
	}
	private static IClasspathEntry getUpdatedEntry(IClasspathEntry currEntry, IClasspathEntry updatedEntry, String[] updatedAttributes, IJavaProject jproject) {
		if (updatedAttributes == null) {
			return updatedEntry; 
		}
		CPListElement currElem= CPListElement.createFromExisting(currEntry, jproject);