title: flutter --flow-control-collections are needed, but are they?
body: <p>After upgrading flutter (both master and stable versions) and dart, I get an error about the experiment --flow-control-collections not being enabled for various for-loops that I'm using in the project. I tried to fix it using <a href="https://stackoverflow.com/questions/55477046/how-can-i-enable-flutter-dart-language-experiments">this entry</a> but that just made things weirder.  So, now I have the below error that tells me that I need the control-flow-collections experiement to be enabled while simultaneously telling me that it's no longer required.</p>
  
  <p><a href="https://i.stack.imgur.com/kN3zV.png" rel="noreferrer"><img src="https://i.stack.imgur.com/kN3zV.png" alt=" eex"></a></p>
  
  <p>This error comes up for every for-loop that I'm using.</p>
  
  <p>Here's my flutter --version result:</p>
  
  <pre><code>Stable:
  Flutter 1.12.13+hotfix.5 • channel stable • https://github.com/flutter/flutter.git
  Framework • revision 27321ebbad (13 days ago) • 2019-12-10 18:15:01 -0800
  Engine • revision 2994f7e1e6
  Tools • Dart 2.7.0
  
  Master:
  Flutter 1.13.6-pre.16 • channel master • https://github.com/flutter/flutter.git
  Framework • revision fcaf9c4070 (2 days ago) • 2019-12-21 14:03:01 -0800
  Engine • revision 33813929e3
  Tools • Dart 2.8.0 (build 2.8.0-dev.0.0 886615d0f9)
  </code></pre>
  
  <p>Any ideas how to resolve this?</p>