build: index file for cdk-experimental/scrolling not being generated

Fixes the `index.js` and `public-api.js` files for the `cdk-experimental/scrolling` packages not being generated. The issue seems to come from the fact that the files aren't reference by any other files which causes them to not be output.

Author: crisbeto

src/cdk-experimental/scrolling/virtual-scroll-viewport.spec.ts

@@ -3,9 +3,7 @@ import {dispatchFakeEvent} from '@angular/cdk/testing';
 import {Component, Input, ViewChild, ViewContainerRef, ViewEncapsulation} from '@angular/core';
 import {ComponentFixture, fakeAsync, flush, TestBed} from '@angular/core/testing';
 import {animationFrameScheduler, Subject} from 'rxjs';
-import {ScrollingModule} from './scrolling-module';
-import {CdkVirtualForOf} from './virtual-for-of';
-import {CdkVirtualScrollViewport} from './virtual-scroll-viewport';
+import {CdkVirtualScrollViewport, CdkVirtualForOf, ScrollingModule} from './index';
 
 
 describe('CdkVirtualScrollViewport', () => {