title: Flutter built value, not generating .g file
body: <p>I am a newbie in flutter. I am building Post class using built-value. The class looks like</p>
  <pre><code>import 'package:built_value/built_value.dart';
  
  part 'posts.g.dart';
  
  abstract class Post implements Built&lt;Post, PostBuilder&gt; {
    Post._();
    factory Post([void Function(PostBuilder) updates]) = _$Post;
  }
  </code></pre>
  <p>With <code>pubspec.yaml</code> file as:</p>
  <pre><code>dependencies:
    url_launcher: &quot;^3.0.0&quot;
    built_value: &quot;^7.1.0&quot;
    built_collection: &quot;^4.3.2&quot;
    flutter:
      sdk: flutter
  
  
    cupertino_icons: &quot;^0.1.3&quot;
  
  dev_dependencies:
    build_runner: &quot;^1.0.0&quot;
    build_test: &quot;^0.10.3&quot;
    flutter_test:
      sdk: flutter
  </code></pre>
  <p>The <code>posts.g.dart</code> file is not generating on running <code>flutter packages pub run build_runner build</code>.</p>