void verifyTickersWereDisposed([String when = 'when none should have been' ])

Throws an exception if any tickers created by the WidgetTester are still active when the method is called.

An argument can be specified to provide a string that will be used in the error message. It should be an adverbial phrase describing the current situation, such as "at the end of the test".

Source 

void verifyTickersWereDisposed([ String when = 'when none should have been' ]) {
  assert(when != null);
  if (_tickers != null) {
    for (Ticker ticker in _tickers) {
      if (ticker.isActive) {
        throw new FlutterError(
          'A Ticker was active $when.\n'
          'All Tickers must be disposed. Tickers used by AnimationControllers '
          'should be disposed by calling dispose() on the AnimationController itself. '
          'Otherwise, the ticker will leak.\n'
          'The offending ticker was: ${ticker.toString(debugIncludeStack: true)}'
        );
      }
    }
  }
}