[md-autofocus] provides an optional way to identify the focused element when a $mdDialog,
$mdBottomSheet, $mdMenu or $mdSidenav opens or upon page load for input-like elements.
When one of these opens, it will find the first nested element with the [md-autofocus]
attribute directive and optional expression. An expression may be specified as the directive
value to enable conditional activation of the autofocus.