Registers a finalization notifier which will be called when the
reference count of closure goes down to 0. Multiple finalization
notifiers on a single closure are invoked in unspecified order. If
a single call to Closure.unref results in the closure being
both invalidated and finalized, then the invalidate notifiers will
be run before the finalize notifiers.
Registers a finalization notifier which will be called when the reference count of closure goes down to 0. Multiple finalization notifiers on a single closure are invoked in unspecified order. If a single call to Closure.unref results in the closure being both invalidated and finalized, then the invalidate notifiers will be run before the finalize notifiers.