tree: invalid for a role=tree to not have any treeitems in the DOM

[Splaktar]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

The Tree component would pass a11y audits out of the box.

What is the current behavior?

Using the current Tree schematic as an example

The DOM includes no elements with role="treeitem". This isn't valid.

What are the steps to reproduce?

https://material.angular.io/components/tree/examples
https://beta-angular-material-io.firebaseapp.com/components/tree/examples

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

https://dequeuniversity.com/rules/axe/2.2/aria-required-children?application=lighthouse
https://www.w3.org/TR/wai-aria/#tree

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular: 6.0.9, 7.0.0-rc.0
Material: 6.4.7, 7.0.0-rc.0
TypeScript: 2.7.2, 3.1.1
Browsers: Chrome, but assumed to affect all

Is there anything else we should know?

The tree becomes valid and passes the audit as soon as it is expanded to a leaf node which will be a role="treeitem".

This issue means that the tree Schematic will generation code that does not pass an a11y audit by default.

[jelbourn]

This may end being something we mainly add documentation around. The tool checks that your structure is correct, but the behavior for what to do when there are legitimately no items is ultimately up to the application. Temporarily hiding the tree is one valid approach, as is adding a "No items" placeholder item to the tree.