IE < 9 does not support CSS content properties, so the +/- does not
get rendered. Instead, add the +/- as text nodes when creating the
DOM nodes for the control.
In IE7, handling some mouseup events are causing the native event
underlying goog.events.BrowserEvent to be invalidated, and it would
error out with a "Member not found" message. By passing the Closure
event rather than the native event around, we avoid this case.