Don't use "objects" in lib/compiler/visitor.js

The "objects" module will be removed.

See #442.

lib/compiler/visitor.js

@@ -1,7 +1,5 @@
 "use strict";
 
-let objects = require("../utils/objects");
-
 /* Simple AST node visitor builder. */
 let visitor = {
   build: function(functions) {
@@ -62,7 +60,11 @@ let visitor = {
           any:          visitNop
         };
 
-    objects.defaults(functions, DEFAULT_FUNCTIONS);
+    Object.keys(DEFAULT_FUNCTIONS).forEach(type => {
+      if (!functions.hasOwnProperty(type)) {
+        functions[type] = DEFAULT_FUNCTIONS[type];
+      }
+    });
 
     return visit;
   }