title: The library 'package:flutter/material.dart' is legacy, and should not be imported into a null safe library
body: <p>I have a null-safe library and in the example folder I'm using the following import:</p>
  <pre><code>import 'package:flutter/material.dart';
  </code></pre>
  <p>However, the linter is giving me the following warning:</p>
  <blockquote>
  <p>The library 'package:flutter/material.dart' is legacy, and should not be imported into a null safe library. Try migrating the imported library. import_of_legacy_library_into_null_safe</p>
  </blockquote>
  <p>The example project's pubspec.yaml file specifies the beta version of the Dart SDK:</p>
  <pre><code>environment:
    sdk: &quot;&gt;=2.12.0-29.10.beta &lt;3.0.0&quot;
  
  dependencies:
    flutter:
      sdk: flutter
    my_library:
      path: ../
  </code></pre>
  <p>Hasn't material.dart already been converted by now? Do I need to add something else to pubspec.yaml or just wait for the stable release to come out?</p>