Test fails occasionally with the report including:
        {
              type: message_received
              value: dart-main-done
              timestamp: 0.142
        },
        {
              type: print
              value: Uncaught Error: Timeout not prevented by events
              timestamp: 1.597
        },

That is, a ~1.5 second delay after which the timeout fires.
This should not happen - the expected behavior is to add error events on the stream controller every 5 ms, 200 times, and then close the controller. The timeout of 500 ms should not have time to happen, and if there is more than 250 ms delay between the controller getting the error and the timeout happening, the test is dropped and won't fail.

Check that the close event isn't dropped somehow.