dialog opened with TemplateRef that contains ng-content is not cleaned up properly on close, causing stacking issues

[fxck]

See https://gist.run/?id=f3741ab47081095d12931e25acb6ce80

1. open dialog
2. select something in autocomplete (optional)
3. close dialog
4. open again
5. click on autocomplete

This stays in dom after closing the dialog for the first time

causing stacking issues the next time it is opened

@crisbeto @jelbourn

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