chore: fix demo-app prod build (#854)

* Currently it was not possible to build the demo-app in production, due to the fact that the fileExtensions for the SystemJS Builder were not available.
  This was caused by the deprecated flag `defaultJSExtension`.

Author: devversion

src/demo-app/system-config.ts

@@ -2,9 +2,6 @@
  * User Configuration.
  **********************************************************************************************/
 
-System.defaultJSExtensions = true;
-
-
 const components = [
   'button',
   'card',
@@ -36,6 +33,11 @@ const packages: any = {
   '@angular2-material/core': {
     format: 'cjs',
     defaultExtension: 'js'
+  },
+  // Set the default extension for the root package, because otherwise the demo-app can't
+  // be built within the production mode. Due to missing file extensions.
+  '.': {
+    defaultExtension: 'js'
   }
 };
 components.forEach(name => {