Override toString in case this function is printed (#10949)

* Override toString in case this function is printed

Related to https://stackoverflow.com/questions/59543968/unexpected-return-value-from-visitor-method

* Don't override `toString` if `wrapper` is true

Override `toString` immediately before returning a newFn, otherwise it will be overridden if wrapper is true

#10949 (comment)

* prettier

packages/babel-traverse/src/visitors.js

@@ -227,6 +227,11 @@ function wrapWithStateOrWrapper(oldVisitor, state, wrapper: ?Function) {
         newFn = wrapper(state.key, key, newFn);
       }
 
+      // Override toString in case this function is printed, we want to print the wrapped function, same as we do in `wrapCheck`
+      if (newFn !== fn) {
+        newFn.toString = () => fn.toString();
+      }
+
       return newFn;
     });