Don't use "objects" in lib/compiler/passes/report-duplicate-labels.js

The "objects" module will be removed.

See #442.

lib/compiler/passes/report-duplicate-labels.js

@@ -1,13 +1,22 @@
 "use strict";
 
 let GrammarError = require("../../grammar-error"),
-    objects      = require("../../utils/objects"),
     visitor      = require("../visitor");
 
 /* Checks that each label is defined only once within each scope. */
 function reportDuplicateLabels(ast) {
+  function cloneEnv(env) {
+    let clone = {};
+
+    Object.keys(env).forEach(name => {
+      clone[name] = env[name];
+    });
+
+    return clone;
+  }
+
   function checkExpressionWithClonedEnv(node, env) {
-    check(node.expression, objects.clone(env));
+    check(node.expression, cloneEnv(env));
   }
 
   let check = visitor.build({
@@ -17,7 +26,7 @@ function reportDuplicateLabels(ast) {
 
     choice: function(node, env) {
       node.alternatives.forEach(alternative => {
-        check(alternative, objects.clone(env));
+        check(alternative, cloneEnv(env));
       });
     },