title: Because testing_app requires SDK version >=2.16.1 <3.0.0, version solving failed. in flutter
body: <p>I am learning more about testing and i want to use <a href="https://pub.dev/packages/mockito" rel="nofollow noreferrer">Mockito Package</a> and <a href="https://pub.dev/packages/build_runner" rel="nofollow noreferrer">Build Runner Packate</a> then i have my class Cat inside lib/models/cat.dart</p>
  <p><a href="https://i.stack.imgur.com/iIjQr.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/iIjQr.jpg" alt="My cat class" /></a></p>
  <p>then i have my lib/test/cat_test.dart file to test cat class</p>
  <p><a href="https://i.stack.imgur.com/fMEqe.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/fMEqe.jpg" alt="cat_test.dart file" /></a></p>
  <p>this is my pubspec.yaml (part of it)</p>
  <p><a href="https://i.stack.imgur.com/Bcnlw.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Bcnlw.jpg" alt="Pubspec.yaml" /></a></p>
  <p>The problem is thrown when i want to run the next command :</p>
  <blockquote>
  <p>dart run build_runner build</p>
  </blockquote>
  <p>and the problem is the next:</p>
  <blockquote>
  <p>Resolving dependencies in /Users/adm/Documents/Things of david/PRACTICAS/Flutter/testing_app...
  The current Dart SDK version is 2.15.1.
  Because testing_app requires SDK version &gt;=2.16.1 &lt;3.0.0, version solving failed.</p>
  </blockquote>
  <p>Flutter doctor:</p>
  <p><a href="https://i.stack.imgur.com/SO0PZ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/SO0PZ.png" alt="Flutter doctor" /></a></p>
  <p>thank you so much for your help.</p>