createTicker method
Creates a ticker with the given callback.
The kind of ticker provided depends on the kind of ticker provider.
Implementation
@override
Ticker createTicker(TickerCallback onTick) {
_tickers ??= <_WidgetTicker>{};
final result = _WidgetTicker(onTick, this,
debugLabel: kDebugMode ? 'created by ${describeIdentity(this)}' : null);
_tickers!.add(result);
return result;
}