mdDialog: if new dialog is opened while another is active, second one can't be hidden

[erikogenvik]

If a new dialog is opened while there's already another dialog being shown, the first one will be hidden and the new one will be shown. However, since 0.11.0 or perhaps 0.11.1 there's something which breaks the second dialog, so that when $mdDialog.hide() is called nothing happens.
The end result is a dialog that can't be removed.

I haven't had time to investigate the cause, but I've added a testcase for this scenario in #PR 5157.

[erikogenvik]

I had some more time, so I pushed a fix in PR #5157.