fix(compiler-cli): avoid handling functions in loadChildren as lazy load routes paths

The change avoids the compiler CLI internal API from mismatching the following case as lazy loading

```
import { NonLazyLoadedModule } from './non-lazy-loaded/non-lazy-loaded.module';

export function getNonLazyLoadedModule() { return NonLazyLoadedModule; }

export const routes = [
{ path: '/some-path', loadChildren: getNonLazyLoadedModule }
];
```

The output of the check is later passed to `RouteDef.fromString()`, so, it makes sense to be only a string.

Fixes angular/angular-cli#3204

modules/@angular/compiler-cli/src/ngtools_impl.ts

@@ -198,7 +198,7 @@ function _collectRoutes(
  */
 function _collectLoadChildren(routes: Route[]): string[] {
   return routes.reduce((m, r) => {
-    if (r.loadChildren) {
+    if (r.loadChildren && typeof r.loadChildren === 'string') {
       return m.concat(r.loadChildren);
     } else if (Array.isArray(r)) {
       return m.concat(_collectLoadChildren(r));