title: Dart, Identifier with exclamation mark in the back
body: <p>Recently I have been developing mobile aplication with flutter, when I looking at the source code for <em>TickerProvider</em> I see these lines:</p>
  <pre><code>mixin SingleTickerProviderStateMixin&lt;T extends StatefulWidget&gt; on State&lt;T&gt; implements TickerProvider {
  Ticker? _ticker;
  
    @override
    Ticker createTicker(TickerCallback onTick) {
      ...
      _ticker = Ticker(onTick, debugLabel: kDebugMode ? 'created by $this' : null);
      return _ticker!;
    }
  
  ...
  }
  </code></pre>
  <p>I'm interested with this line:</p>
  <pre><code>return _ticker!;
  </code></pre>
  <p>I have seen boolean identifier with exclamation mark in the front meaning that it will return the opposite value of it, but I never see this one. Can someone tell me what this does?</p>