How to dismiss an AlertDialog on a FlatButton click?
worked for me, but the Flutter Team's gallery contains an example using:
Navigator.of(context, rootNavigator: true).pop()
which also works, and I am tempted to follow their lead.
Navigator.pop() should do the trick. You can also use that to return the result of the dialog (if it presented the user with choices)
Navigator.of(context, rootNavigator: true).pop('dialog')
worked with me.