Trigger + pop-up controls are not in DOM order for VoiceOver navigation

#### Bug, feature request, or proposal:
bug

#### What is the expected behavior?
User can use VO-down arrow to navigate the drop down list
user can use VO-space to select one value from drop down list

#### What is the current behavior?
user can't reach the dropdown list using VO command

#### What are the steps to reproduce?
Providing a StackBlitz reproduction is the *best* way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

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

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
angular material 2 as of 2018-05-03

#### Is there anything else we should know?