Replaced core-js default import in favour of specific deps import (PatrickJS#505)

Author: jmsegrev

src/polyfills.ts

@@ -7,7 +7,8 @@
 // import 'es7-reflect-metadata';
 
 // Prefer CoreJS over the polyfills above
-import 'core-js';
+import 'core-js/es6';
+import 'core-js/es7/reflect';
 require('zone.js/dist/zone');
 
 if ('production' === ENV) {