tree: importing MatIconModule and MatButtonModule is necessary

[tom-richter]

It would be helpful to mention in the API reference for Angular Material tree that besides the MatTreeModule also MatIconModule and MatButtonModule must be imported into the app.module.ts.
(eg. when you use a "Tree with flat nodes")

import {MatButtonModule} from '@angular/material/button';
import {MatIconModule} from '@angular/material/icon';
import {MatTreeModule} from '@angular/material/tree';

[Splaktar]

As this is docs' content, I'll move this over the the Components repo. This repo is only for docs infrastructure.

[crisbeto]

For what it's worth, the icon and button modules aren't mandatory for the tree to work, but they are needed if you wanted to replicate the demo from the docs. We should still mention it somewhere in the docs.

[0x1211]

@crisbeto Considering most of the components usage examples are using mat-icon-button directive or <mat-icon> selector would this still be relevant (another example being the Menu component) ? Should some kind of global mention to these necessary imports be considered in the documentation then ?

[tom-richter]

Yes, for me as an Angular Material beginner I copied the code from the documentation and spent half an hour debugging until I found out why the material tree style is not applied. There is no descriptive error message. The tree just looks strange like in this issue:
https://stackoverflow.com/questions/57527296/material-tree-style-not-being-applied/63174068#63174068

[zarend]

Closing this because importing the mat button and icon modules are not required for the tree to work. In the case of the flat tree example, we are directing using MatButton, and we directly use MatIcon inside the button. It's the direct usages that require the MatIconModule and the MatButtonModule. In other cases where a component is used inside another (like MatTreeNode), the dependencies are already imported in the appropriate NgModule.

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