title: Dart build runner generate one dart file with content
body: <p>I am working on a dart package with includes over 200 models and at the moment i have to write manually one line of "export" for each model, to make the models available for everyone who uses this package.</p>
  
  <p>I want the build runner to generate one dart file which contains every export definition.</p>
  
  <p>Therefore I would create an annotation "ExportModel". The builder should search for each class annotated with this annotation.</p>
  
  <p>I tried creating some Builders, but they will generate a *.g.dart file for each class that is annotated. I just want to have one file.</p>
  
  <p>Is where a way to create a builder that runs only once and creates a file at the end ?</p>