Smoke test failed: "A Timer is still pending even after the widget
tree was disposed." Both workers fired their initial-delay Timer
via `Timer(duration, _sweep)` and stored only the periodic ticker
in the cancellable field — the one-shot Timer leaked past dispose
and tripped the test framework's invariant check.
Track both as _initialTimer + _intervalTimer; cancel both in
dispose(). Behavior is unchanged in production (ref.onDispose only
fires on process death normally); this is purely a test-harness
fix.
bvandeusen
d27dd69bfc