CdkTreeNodeOutlet can't be put inside a directive

[igor-cz]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

1. Create nested cdk tree
2. Add custom directive to the parent div of cdkTreeNodeOutlet

What is the current behavior?

Only top level nodes are displayed.

What are the steps to reproduce?

https://stackblitz.com/edit/angular-7pnlug

Is there anything else we should know?

CdkNestedTreeNode has @ContentChildren(CdkTreeNodeOutlet) - changing it to @ContentChildren(CdkTreeNodeOutlet, { descendants: true }) solves the issue