Fix for named-register extra. Resolves systemjs#2040. (systemjs#2042)

* Fix for named-register extra. Resolves systemjs#2040.

* Guy's feedback

src/extras/named-register.js

@@ -6,22 +6,26 @@
  * System.register('x', ...) can be imported as System.import('x')
  */
 (function () {
+  setRegisterRegistry(System);
   const systemJSPrototype = System.constructor.prototype;
-
   const constructor = System.constructor;
   const SystemJS = function () {
     constructor.call(this);
-    this.registerRegistry = Object.create(null);
+    setRegisterRegistry(this);
   };
   SystemJS.prototype = systemJSPrototype;
-  System = new SystemJS();
+  System.constructor = SystemJS;
 
   let firstNamedDefine;
 
   function clearFirstNamedDefine () {
     firstNamedDefine = null;
   }
 
+  function setRegisterRegistry(systemInstance) {
+    systemInstance.registerRegistry = Object.create(null);
+  }
+
   const register = systemJSPrototype.register;
   systemJSPrototype.register = function (name, deps, declare) {
     if (typeof name !== 'string')