[MdAppDrawer] TypeError: child.data.attrs is undefined

[poulpe38000]

Steps to reproduce

Use a simple MdApp structure with a drawer

<md-app md-mode="fixed" style="height: 100vh;" md-waterfall>

    <md-app-toolbar class="md-primary">
    ...
    </md-app-toolbar>

    <md-app-drawer>
    ...
    </md-app-drawer>

    ...
</md-app>

Without setting at least md-persistent or md-permanent on md-app-drawer, you will get a render error

Which browser?

Bug is present on Chrome 66.0.3359.139 and Firefox 59.0.3

What is expected?

MdAppDrawer should work without setting either of those 2 attributes.

What is actually happening?

Issue lies in the buildSlots function of MdApp.vue, especially this section

child.key = JSON.stringify({

  'persistent': child.data.attrs['md-persistent'],

  'permanent': child.data.attrs['md-permanent']

})

If no attributes are defined, child.data.attrs is undefined, and will throw an error if trying to access its child attributes.