title: flutter chopper build fail after upgrade from 3.0.1 => 4.0.1, please help to understand generator output
body: <p>I did upgrade my flutter project to flutter2, as a part of the upgrade I did upgrade chopper version from 3.0.1 to 4.0.1 nullsafety. After that the command <code>flutter packages pub run build_runner watch --delete-conflicting-outputs</code> start to show errors which is very hard to understand because I cannot even find the code which is the error messages points to.
  Here is the error output fragment:</p>
  <pre><code>[INFO] 16.1s elapsed, 35/51 actions completed.
  [SEVERE] chopper_generator:chopper_generator on lib/data/api/my_api.dart:
  
  Could not format because the source could not be parsed:
  
  line 29, column 53 of .: Expected to find ')'.
     ╷
  29 │ final $parts = &lt;PartValue&gt;[PartValue&lt;String*&gt;('name', name), PartValueFile&lt;MultipartFile&gt;('file', file)];
     │                                                     ^
     ╵
  line 124, column 63 of .: Expected to find '}'.
      ╷
  124 │ @override Future&lt;Response&lt;BuiltList&lt;Places&gt;&gt;&gt; getPlace({String* jwtToken}) { final $url = 'place';
      │                                                               ^
      ╵
  line 55, column 55 of .: Expected to find ')'.
  </code></pre>
  <p>In the source code lib/data/api/my_api.dart at 29 line, there is nothing like the message showing, but there is: <a href="https://i.stack.imgur.com/bYvbf.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/bYvbf.png" alt="enter image description here" /></a></p>
  <p>I did search all project files and have not found any files containing code error messages showing me. Looking at the variable types and names it can be assumed the lines, matching the errors in the my_api.dart code is:</p>
  <pre><code>  @multipart
    Future&lt;Response&lt;Users&gt;&gt; postMe({
      @Header('Authorization') String jwtToken,
      @Part(&quot;name&quot;) String name,
      @PartFile(&quot;file&quot;) MultipartFile file
    });  
  </code></pre>
  <p>But I cannot figure out whats wrong with them?</p>
  <p>---update---</p>
  <p>Found an answer <a href="https://www.gitmemory.com/issue/lejard-h/chopper/249/824860498" rel="nofollow noreferrer">https://www.gitmemory.com/issue/lejard-h/chopper/249/824860498</a>
  I have generated my chopper api after adding this to pubspec.yaml</p>
  <pre><code>environment:
    sdk: &quot;&gt;=2.12.0 &lt;3.0.0&quot;
  
  dependency_overrides:
    analyzer: ^1.4.0
    hive_generator:
    quiver: ^3.0.1 
  </code></pre>
  <p>But this is not a solution for me because my project is not nullsafey, It is using sdk&gt;=2.9.0. It seems there is only the hard way, it is to do nullsafety migration((. Thanks</p>