private Set<String> revertRenamingOfExternalClasses(CssTree cssTree, Map<String,
String> styleClassesMapping) {
ExternalClassesCollector externalClassesCollector = new ExternalClassesCollector(cssTree
.getMutatingVisitController(), styleClassesMapping.keySet());
externalClassesCollector.runPass();
Set<String> externalClasses = externalClassesCollector.getExternalClassNames();
final Map<String, String> revertMap = new HashMap<String, String>(externalClasses.size());