1. `evt` -> `_`
2. Could you try using `asyncFire('cancelled')` here instead, and listening to that at the usage site as `progressComponent.on['cancelled'].listen((e) { ... })`? Also, that's not relevant here, but if an event payload needs to be passed, that can be done as `asyncFire('cancelled', detail: {var1: val1, var2: val2})` and accessed by the listener via `e.detail`. Using asyncFire is idiomatic Polymer, and it will allow other users of the control to attach a listener to the event directly in the HTML as `<spark-progress on-cancelled="{{cancelledHandler}}">`.