Merge pull request #13346 from unional/unional-systemjs

Unional systemjs

systemjs/index.d.ts

@@ -257,7 +257,7 @@ declare namespace SystemJSLoader {
         /**
          * This represents the System base class, which can be extended or reinstantiated to create a custom System instance.
          */
-        constructor(): System;
+        constructor: new () => System;
 
         /**
          * Deletes a module from the registry by normalized name.

systemjs/systemjs-tests.ts

@@ -35,3 +35,5 @@ System.import('./app.js').then(function (m) {
 System.import('lodash').then(function (_) {
     console.log(_);
 });
+
+const clonedSystemJS = new System.constructor();