Mat-tree :Tree with nested nodes adding child doesnt update the view.

#### Bug:

When using mat-nested-tree-node with a nested datasource if you add a child to an existing parent it does not update the view.

#### What is the expected behavior?
Well if you change or add a child to an existing parent in the nestedDataSource it should update the tree.

#### What is the current behavior?
It does not update the view when adding a child to an existing parent to the nestedDataStructure .

#### What is the use-case or motivation for changing an existing behavior?

Well in my case i need to update the view since i add/delete nodes .

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
versions 6 (Chrome ver 66)

#### Is there anything else we should know?
I found an workaround:
refreshTree(){
let _data = this.nestedDataSource.data;
this.nestedDataSource.data = null;
this.nestedDataSource.data = _data;
}

Start with mat-nested-tree-node and the nestedDataSource update path described in the issue, reproducing the problem by adding a child to an existing parent. Compare the view with the documented refreshTree workaround; done means the nested tree reflects added or deleted nodes without that workaround.

Written by the indexing model from the issue text.