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 ??= <_CustomTicker>{};
final _CustomTicker result = _CustomTicker(onTick, this, debugLabel: 'created by $this');
_tickers!.add(result);
return result;
}