make experimental transformers declare that they're experimental

lib/6to5/transformation/transformers/es7-abstract-references.js

@@ -3,6 +3,8 @@
 var util = require("../../util");
 var t    = require("../../types");
 
+exports.experimental = true;
+
 var container = function (parent, call, ret) {
   if (t.isExpressionStatement(parent)) {
     // we don't need to worry about return values

lib/6to5/transformation/transformers/es7-array-comprehension.js

@@ -1,6 +1,8 @@
 var util = require("../../util");
 var t    = require("../../types");
 
+exports.experimental = true;
+
 var single = function (node, file) {
   var block = node.blocks[0];
 

lib/6to5/transformation/transformers/es7-exponentiation-operator.js

@@ -1,5 +1,7 @@
 // https://github.com/rwaldron/exponentiation-operator
 
+exports.experimental = true;
+
 var t = require("../../types");
 var pow = t.memberExpression(t.identifier("Math"), t.identifier("pow"));
 

lib/6to5/transformation/transformers/es7-generator-comprehension.js

@@ -1,6 +1,8 @@
 var arrayComprehension = require("./es7-array-comprehension");
 var t                  = require("../../types");
 
+exports.experimental = true;
+
 exports.ComprehensionExpression = function (node) {
   if (!node.generator) return;
 

lib/6to5/transformation/transformers/es7-object-spread.js

@@ -2,6 +2,8 @@
 
 var t = require("../../types");
 
+exports.experimental = true;
+
 exports.ObjectExpression = function (node) {
   var hasSpread = false;
   var i;