put experimental ES7 features behind a flag --experimental and experimental option

lib/6to5/file.js

@@ -25,15 +25,16 @@ File.normaliseOptions = function (opts) {
   opts = _.cloneDeep(opts || {});
 
   _.defaults(opts, {
-    whitespace: true,
-    blacklist:  [],
-    whitelist:  [],
-    sourceMap:  false,
-    comments:   true,
-    filename:   "unknown",
-    modules:    "common",
-    runtime:    false,
-    code:       true
+    experimental: false,
+    whitespace:   true,
+    blacklist:    [],
+    whitelist:    [],
+    sourceMap:    false,
+    comments:     true,
+    filename:     "unknown",
+    modules:      "common",
+    runtime:      false,
+    code:         true
   });
 
   // normalise windows path separators to unix

lib/6to5/register.js

@@ -57,24 +57,25 @@ blacklistTest("unicodeRegex", function () { new RegExp("foo", "u"); });
 
 //
 
-var ignoreRegex = /node_modules/;
+var transformOpts = {};
+var ignoreRegex   = /node_modules/;
 var onlyRegex;
-var whitelist   = [];
-var exts        = {};
-var maps        = {};
-var old         = require.extensions[".js"];
+var whitelist     = [];
+var exts          = {};
+var maps          = {};
+var old           = require.extensions[".js"];
 
 var loader = function (m, filename) {
   if ((ignoreRegex && ignoreRegex.test(filename)) || (onlyRegex && !onlyRegex.test(filename))) {
     return old.apply(this, arguments);
   }
 
-  var result = to5.transformFileSync(filename, {
+  var result = to5.transformFileSync(filename, _.extend({
     whitelist: whitelist,
     blacklist: blacklist,
     sourceMap: true,
     modules:   "commonInterop"
-  });
+  }, transformOpts));
 
   maps[filename] = result.map;
 
@@ -107,6 +108,5 @@ module.exports = function (opts) {
 
   if (opts.extensions) hookExtensions(util.arrayify(opts.extensions));
 
-  if (opts.blacklist) blacklist = util.arrayify(opts.blacklist);
-  if (opts.whitelist) whitelist = util.arrayify(opts.whitelist);
+  _.extend(transformOpts, opts);
 };

lib/6to5/util.js

@@ -218,7 +218,7 @@ exports.parse = function (opts, code, callback) {
     var ast = acorn.parse(code, {
       allowReturnOutsideFunction: true,
       preserveParens:             true,
-      ecmaVersion:                Infinity,
+      ecmaVersion:                opts.experimental ? 7 : 6,
       strictMode:                 true,
       onComment:                  comments,
       locations:                  true,