If name_cache is specified, do rename cached properties

(even if --mangle-props is not there)

lib/propmangle.js

@@ -63,7 +63,8 @@ function find_builtins() {
 function mangle_properties(ast, options) {
     options = defaults(options, {
         reserved : null,
-        cache : null
+        cache : null,
+        only_cache : false
     });
 
     var reserved = options.reserved;
@@ -139,6 +140,9 @@ function mangle_properties(ast, options) {
     // only function declarations after this line
 
     function can_mangle(name) {
+        if (options.only_cache) {
+            return cache.props.has(name);
+        }
         if (reserved.indexOf(name) >= 0) return false;
         if (/^[0-9.]+$/.test(name)) return false;
         return true;