anchor.diff:  
     void tearDown() {
       server.done();
  -    server = null;
  -    serverChannel = null;
     }
   
     /// Returns a [Future] that completes when the server's analysis is complete.
  
positive.code:   migrationTest.setUp();
    print('Migrating');
    await migrationTest.run(packageRoot, port);
    if (port == null) {
      print('Done');
      io.exit(0);
    } else {
      print('Done.  Please point your browser to localhost:$port/\$filePath');
    }
  }
  
  class MigrationBase {
    ResourceProvider resourceProvider = PhysicalResourceProvider.INSTANCE;
    MockServerChannel serverChannel;
    AnalysisServer server;
  
    AnalysisServer createAnalysisServer() {
      //
  
negative.code:     } else if (builtInPackages.contains(packageName)) {
        return path.join(sdkRoot, 'pkg', packageName);
      } else if (externalPackages.containsKey(packageName)) {
        return externalPackages[packageName] as String;
      } else {
        throw StateError('Unrecognized package $packageName');
      }
  