add global symbol registry to polyfill

lib/6to5/polyfill.js

@@ -1,7 +1,7 @@
 /* jshint newcap: false */
 
 var ensureSymbol = function (key) {
-  Symbol[key] = Symbol[key] || Symbol();
+  Symbol[key] = Symbol[key] || Symbol(key);
 };
 
 var ensureProto = function (Constructor, key, val) {
@@ -13,11 +13,25 @@ var ensureProto = function (Constructor, key, val) {
 
 if (typeof Symbol === "undefined") {
   require("es6-symbol/implement");
+
+  var globSymbols = {};
+
+  Symbol.for = function (key) {
+    return globSymbols[key] = globSymbols[key] || Symbol(key);
+  };
+
+  Symbol.keyFor = function (sym) {
+    return sym.__description__;
+  };
 }
 
 require("es6-shim");
 require("./transformation/transformers/es6-generators/runtime");
 
+ensureSymbol("species");
+
+String.prototype.includes = String.prototype.includes || String.prototype.contains;
+
 // Abstract references
 
 ensureSymbol("referenceGet");