fix issues with closing dialogs

Partially roll back a previous CL (https://github.com/dart-lang/spark/pull/1995/files). This caused issues closing dialogs. I'm not sure what the issue is, but it looks like listening on the `a.close` elements causes their `overlay-toggle` behavior to not work.

There are still problems with `Close` on the about box and the settings dialog (https://github.com/dart-lang/spark/issues/2028).

@ussuri