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/preferences/ComplianceConfigurationBlock.java
public void widgetSelected(SelectionEvent e) {
				widgetDefaultSelected(e);
			}
		});
		GridData gd= new GridData(GridData.FILL, GridData.FILL, true, true);
		gd.widthHint= fPixelConverter.convertWidthInCharsToPixels(50);
		fJRE50InfoText.setLayoutData(gd);
		validateJRE50Status();
		
		return sc1;
	}
	
	protected final void openBuildPathPropertyPage() {
		if (getPreferenceContainer() != null) {
			Map data= new HashMap();
			data.put(BuildPathsPropertyPage.DATA_REVEAL_ENTRY, JavaRuntime.getDefaultJREContainerEntry());
			getPreferenceContainer().openPage(BuildPathsPropertyPage.PROP_ID, data);
		}
		validateJRE50Status();
	}
	
	protected final void openJREInstallPreferencePage() {
		String jreID= BuildPathSupport.JRE_PREF_PAGE_ID;
		if (fProject == null && getPreferenceContainer() != null) {
			getPreferenceContainer().openPage(jreID, null);
		} else {
			PreferencesUtil.createPreferenceDialogOn(getShell(), jreID, new String[] { jreID }, null).open();
		}
		validateJRE50Status();
	}