add missing use strict declarations

lib/6to5/helpers/clone.js

@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = function cloneDeep(obj) {
   var obj2 = {};
   if (!obj) return obj2;

lib/6to5/transformation/transformers/es6/block-scoping-tdz.js

@@ -1,3 +1,5 @@
+"use strict";
+
 var traverse = require("../../../traverse");
 var t        = require("../../../types");
 

lib/6to5/transformation/transformers/validation/undeclared-variable-check.js

@@ -1,3 +1,5 @@
+"use strict";
+
 exports.optional = true;
 
 exports.Identifier = function (node, parent, scope, context, file) {

lib/6to5/types/node.js

@@ -1,3 +1,5 @@
+"use strict";
+
 module.exports = Node;
 
 var acorn = require("acorn-6to5");