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
private static Key[] getKeys() {
		return new Key[] {
				PREF_LOCAL_VARIABLE_ATTR, PREF_LINE_NUMBER_ATTR, PREF_SOURCE_FILE_ATTR, PREF_CODEGEN_UNUSED_LOCAL,
				PREF_CODEGEN_INLINE_JSR_BYTECODE,
				PREF_COMPLIANCE, PREF_SOURCE_COMPATIBILITY,
				PREF_CODEGEN_TARGET_PLATFORM, PREF_PB_ASSERT_AS_IDENTIFIER, PREF_PB_ENUM_AS_IDENTIFIER
			};
	}
		
	/* (non-Javadoc)
	 * @see org.eclipse.jdt.internal.ui.preferences.OptionsConfigurationBlock#settingsUpdated()
	 */
	protected void settingsUpdated() {
		setValue(INTR_DEFAULT_COMPLIANCE, getCurrentCompliance());
		super.settingsUpdated();
	}
	
	
	/*
	 * @see org.eclipse.jface.preference.PreferencePage#createContents(Composite)
	 */
	protected Control createContents(Composite parent) {
		fPixelConverter= new PixelConverter(parent);
		setShell(parent.getShell());
		
		Composite complianceComposite= createComplianceTabContent(parent);
		
		validateSettings(null, null, null);