add bindingEquals, typeEquals, referenceEquals helper methods to Scope

lib/6to5/transformation/transformers/es6/constants.js

@@ -24,8 +24,8 @@ var visitor = {
         // constant so we can just ignore it
         if (id === constant) continue;
 
-        var localBinding = scope.getBinding(key);
-        if (localBinding !== constant) continue;
+        // check if there's been a local binding that shadows this constant
+        if (scope.bindingEquals(key, constant)) continue;
 
         throw state.file.errorWithNode(id, key + " is read-only");
       }

lib/6to5/traversal/scope.js

@@ -492,6 +492,10 @@ each({
   binding: "Binding",
   type: "Type"
 }, function (title, type) {
+  Scope.prototype[type + "Equals"] = function (id, node) {
+    return this["get" + title](id) === node;
+  };
+
   each([
     "get",
     "has",