fix(bundles): correct RxJS mapping in rollup config for umd/es5 bundles

modules/@angular/core/rollup-testing.config.js

@@ -6,10 +6,8 @@ export default {
   moduleName: 'ng.core.testing',
   globals: {
     '@angular/core': 'ng.core',
-    'rxjs/Subject': 'Rx',
-    'rxjs/observable/PromiseObservable': 'Rx', // this is wrong, but this stuff has changed in rxjs b.6 so we need to fix it when we update.
-    'rxjs/operator/toPromise': 'Rx.Observable.prototype',
-    'rxjs/Observable': 'Rx'
+    'rxjs/Observable': 'Rx',
+    'rxjs/Subject': 'Rx'
   }
 }