don't run playground transformers at all if playground isn't enabled

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

@@ -31,7 +31,7 @@ var visitor = {
 
 exports.Scope = function (node, parent, scope, context, file) {
   traverse(node, visitor, scope, {
-    constants: scope.getAllOfKind("const"),
+    constants: scope.getAllDeclarationsOfKind("const"),
     file:      file
   });
 };

lib/6to5/transformation/transformers/playground/mallet-operator.js

@@ -1,7 +1,9 @@
 "use strict";
 
 var build = require("../../helpers/build-conditional-assignment-operator-transformer");
-var t      = require("../../../types");
+var t     = require("../../../types");
+
+exports.playground = true;
 
 build(exports, {
   is: function (node, file) {

lib/6to5/transformation/transformers/playground/memoization-operator.js

@@ -3,6 +3,8 @@
 var build = require("../../helpers/build-conditional-assignment-operator-transformer");
 var t     = require("../../../types");
 
+exports.playground = true;
+
 build(exports, {
   is: function (node) {
     var is = t.isAssignmentExpression(node) && node.operator === "?=";

lib/6to5/transformation/transformers/playground/method-binding.js

@@ -2,6 +2,8 @@
 
 var t = require("../../../types");
 
+exports.playground = true;
+
 exports.BindMemberExpression = function (node, parent, scope) {
   var object = node.object;
   var prop   = node.property;

lib/6to5/transformation/transformers/playground/object-getter-memoization.js

@@ -3,6 +3,8 @@
 var traverse = require("../../../traverse");
 var t        = require("../../../types");
 
+exports.playground = true;
+
 var visitor = {
   enter: function (node, parent, scope, context, state) {
     if (t.isFunction(node)) return;