squash: do not mutate functions

Author: LiviaMedeiros

test/common/index.js

@@ -522,7 +522,8 @@ function mustNotCall(msg) {
 const _mustNotMutateProxies = new WeakMap();
 
 function mustNotMutate(object) {
-  if (object === null || typeof object !== 'object')
+  if (object === null ||
+      typeof object !== 'object' && typeof object !== 'function')
     return object;
 
   const cachedProxy = _mustNotMutateProxies.get(object);