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
/* (non-javadoc)
	 * Update fields and validate.
	 * @param changedKey Key that changed, or null, if all changed.
	 */	
	protected void validateSettings(Key changedKey, String oldValue, String newValue) {
		if (!areSettingsEnabled()) {
			return;
		}
		if (changedKey != null) {
			if (INTR_DEFAULT_COMPLIANCE.equals(changedKey)) {
				updateComplianceEnableState();
				updateComplianceDefaultSettings(true, null);
				fComplianceStatus= validateCompliance();
			} else if (PREF_COMPLIANCE.equals(changedKey)) {
			    
			    Object oldDefault= setValue(INTR_DEFAULT_COMPLIANCE, DEFAULT_CONF);
			    updateComplianceEnableState();
				updateComplianceDefaultSettings(USER_CONF.equals(oldDefault), oldValue);
				fComplianceStatus= validateCompliance();
				validateJRE50Status();
			} else if (PREF_SOURCE_COMPATIBILITY.equals(changedKey)) {
				updateAssertEnumAsIdentifierEnableState();
				fComplianceStatus= validateCompliance();
			} else if (PREF_CODEGEN_TARGET_PLATFORM.equals(changedKey)) {
				updateInlineJSREnableState();
				fComplianceStatus= validateCompliance();
			} else if (PREF_PB_ENUM_AS_IDENTIFIER.equals(changedKey) ||
					PREF_PB_ASSERT_AS_IDENTIFIER.equals(changedKey)) {
				fComplianceStatus= validateCompliance();
			} else {