add support for exit traversal functions to replace the node

lib/6to5/traverse/index.js

@@ -34,20 +34,30 @@ var traverse = module.exports = function (parent, callbacks, blacklistTypes) {
       // type is blacklisted
       if (_.contains(blacklistTypes, node.type)) return;
 
-      // enter
-      var result = callbacks.enter(node, parent, obj, key);
-
-      // stop iteration
-      if (result === false) return;
+      var result;
 
       // replace node
-      if (result != null) node = obj[key] = result;
+      var maybeReplace = function (result) {
+        if (result != null) obj[key] = result;
+      };
+
+      // enter
+      if (callbacks.enter) {
+        result = callbacks.enter(node, parent, obj, key);
+
+        // stop iteration
+        if (result === false) return;
+
+        maybeReplace(result);
+      }
 
       // traverse node
       traverse(node, callbacks, blacklistTypes);
 
       // exit
-      if (callbacks.exit) callbacks.exit(node, parent, obj, key);
+      if (callbacks.exit) {
+        maybeReplace(callbacks.exit(node, parent, obj, key));
+      }
     };
 
     if (_.isArray(nodes)) {