title: pubspec.yaml has no lower-bound SDK constraint
body: <p>I was taking MDC101 flutter code lab. I cloned the starter project from the git repository as per the instructions but after clonning done, I executed <code>flutter pub get</code> and it gave me the following error.</p>
  <pre><code>pubspec.yaml has no lower-bound SDK constraint.
  You should edit pubspec.yaml to contain an SDK constraint:
  environment:
    sdk: '&gt;=2.10.0 &lt;3.0.0'
  </code></pre>