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
IClasspathEntry curr= oldClasspath[i];
			if (curr.getEntryKind() == entryKind && curr.getPath().equals(jarPath)) {
				
				newEntries.add(getUpdatedEntry(curr, newEntry, changedAttributes, jproject));
				found= true;
			} else {
				newEntries.add(curr);
			}
		}
		if (!found) {
			if (!putJarOnClasspathDialog(shell)) {
				return;
			}
			
			newEntries.add(newEntry);			
		}
		IClasspathEntry[] newClasspath= (IClasspathEntry[]) newEntries.toArray(new IClasspathEntry[newEntries.size()]);
		jproject.setRawClasspath(newClasspath, monitor);
	}
	
	private static boolean putJarOnClasspathDialog(final Shell shell) {
		if (shell == null) {
			return false;
		}
		
		final boolean[] result= new boolean[1];
		shell.getDisplay().syncExec(new Runnable() {
			public void run() {
				String title= NewWizardMessages.BuildPathSupport_putoncpdialog_title; 
				String message= NewWizardMessages.BuildPathSupport_putoncpdialog_message;