title: AnimationController The named parameter 'vsync' isn't defined
body: <p>AnimationController stopped working because somehow vsync is not a named parameter anymore.</p>
  <p>This line of code stopped working.</p>
  <p><code>controller = AnimationController(duration: Duration(seconds: 3), vsync: this);</code></p>
  <p>It now shows an error saying:</p>
  <pre><code>The named parameter 'vsync' isn't defined.
  Try correcting the name to an existing named parameter's name, or defining a named parameter with the name 'vsync'.dartundefined_named_parameter
  </code></pre>
  <p>I use this exact same code in two different apps, in one of them I just removed the fading text animation as a hot fix, but on the other app I need a real fix. Has anyone seen this problem recently?</p>
  <p>Note:</p>
  <p>-This exact same code has been working for months and stopped working after an update.</p>
  <p>-The surrounding class does have: <code>with TickerProviderStateMixin</code></p>
  <pre><code>class FadingText extends StatefulWidget {
    final String text;
    final int seconds;
    final TextStyle style;
  
    FadingText({this.text, this.seconds, this.style});
  
    @override
    _FadingTextState createState() =&gt; _FadingTextState();
  }
  
  class _FadingTextState extends State&lt;FadingText&gt; with TickerProviderStateMixin {
    AnimationController controller;
    Animation animation;
  
    @override
    Widget build(BuildContext context) {
      return Container(
        child: FadeTransition(
          opacity: animation,
          child: Text(widget.text, style: widget.style,),
        ),
      );
    }
  
    @override
    void initState() {
      super.initState();
  
      controller = AnimationController(duration: Duration(seconds: widget.seconds), vsync: this);
      animation = Tween(begin: 0.5, end: 1.0).animate(controller);
  
      animation.addStatusListener((status) {
        if (status == AnimationStatus.completed) { controller.reverse(); }
        else if (status == AnimationStatus.dismissed) { controller.forward(); }
      });
  
      controller.forward();
    }
  
  
    @override
    void dispose() {
      controller.dispose();
      super.dispose();
    }
  
  }
  </code></pre>
  <pre><code>Flutter Doctor
  
  
  [✓] Flutter (Channel master, 1.21.0-6.0.pre.140, on Mac OS X 10.15.5 19F101, locale en-MX)
      • Flutter version 1.21.0-6.0.pre.140 at /Users/luisharo/Developer/flutter
      • Framework revision 7884420a0a (25 hours ago), 2020-07-31 20:20:00 +0200
      • Engine revision 280bbfc763
      • Dart version 2.10.0 (build 2.10.0-2.0.dev bd528bfbd6)
  
   
  [✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
      • Android SDK at /Users/luisharo/Library/Android/sdk
      • Platform android-29, build-tools 29.0.3
      • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
      • Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b4-5784211)
      • All Android licenses accepted.
  
  [✓] Xcode - develop for iOS and macOS (Xcode 11.5)
      • Xcode at /Applications/Xcode.app/Contents/Developer
      • Xcode 11.5, Build version 11E608c
      • CocoaPods version 1.8.4
  
  [✓] Chrome - develop for the web
      • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome
  
  [✓] Android Studio (version 3.6)
      • Android Studio at /Applications/Android Studio.app/Contents
      • Flutter plugin version 45.1.1
      • Dart plugin version 192.7761
      • Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b4-5784211)
  
  [✓] IntelliJ IDEA Community Edition (version 2019.2.4)
      • IntelliJ at /Applications/IntelliJ IDEA CE.app
      • Flutter plugin version 45.1.1
      • Dart plugin version 192.8052
  
  [✓] VS Code (version 1.47.3)
      • VS Code at /Applications/Visual Studio Code.app/Contents
      • Flutter extension version 3.13.1
  
  [✓] Connected device (4 available)
      • JKM LX3 (mobile)       • 7MLNW19723003608                     • android-arm64  • Android 9 (API 28)
      • iPhone 11 Pro (mobile) • 675C24C4-7682-4DFE-8037-062893405EE7 • ios            • com.apple.CoreSimulator.SimRuntime.iOS-13-5
        (simulator)
      • Web Server (web)       • web-server                           • web-javascript • Flutter Tools
      • Chrome (web)           • chrome                               • web-javascript • Google Chrome 84.0.4147.105
  
  • No issues found!
  </code></pre>