bug(menu): Error Host already has a portal attached

[cladera]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

When open a md-menu programatically using trigger's api it should not throw an exception

What is the current behavior?

When a menu is opened and closed several times it eventually throws an exception:

browser_adapter.js:95 Error: Uncaught (in promise): Error: Host already has a portal attached
    at resolvePromise (zone.js:538)
    at resolvePromise (zone.js:523)
    at zone.js:571
    at ZoneDelegate.invokeTask (zone.js:356)
    at Object.onInvokeTask (ng_zone_impl.js:55)
    at ZoneDelegate.invokeTask (zone.js:355)
    at Zone.runTask (zone.js:256)
    at drainMicroTaskQueue (zone.js:474)
    at HTMLInputElement.ZoneTask.invoke (zone.js:426)
    at MdInput.focus (input.js:202)

What are the steps to reproduce?

http://plnkr.co/edit/1KDM4LwHtlpJQAUbAuur?p=preview

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

I'd want to be able to open and close a menu not only with click event but other events such as "focus".

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

Angular 2.0.0-rc.5 Material 1.0.0-alpha.7-4

Is there anything else we should know?

Besides the error, the plunker does not behave as expected. When using "focus" handler the menu opens but immediately closes. It is necessary to click the input again to open the menu. In my application this does not happen.

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