buildDartApplication: include dart SDK builder by default

  generators = callPackage ./generators.nix { inherit dart; } { buildDrvArgs = args; };

  pubspecLockFile = builtins.toJSON pubspecLock;

  pubspecLockData = pub2nix.readPubspecLock { inherit src packageRoot pubspecLock gitHashes sdkSourceBuilders customSourceBuilders; };

  pubspecLockData = pub2nix.readPubspecLock {

    inherit src packageRoot pubspecLock gitHashes customSourceBuilders;

    sdkSourceBuilders  = {

      # https://github.com/dart-lang/pub/blob/e1fbda73d1ac597474b82882ee0bf6ecea5df108/lib/src/sdk/dart.dart#L80

      "dart" = name: runCommand "dart-sdk-${name}" { passthru.packageRoot = "."; } ''

        for path in '${dart}/pkg/${name}'; do

          if [ -d "$path" ]; then

            ln -s "$path" "$out"

            break

          fi

        done

        if [ ! -e "$out" ]; then

          echo 1>&2 'The Dart SDK does not contain the requested package: ${name}!'

          exit 1

        fi

      '';

    } // sdkSourceBuilders;

  };

  packageConfig = generators.linkPackageConfig {

    packageConfig = pub2nix.generatePackageConfig {

      pname = if args.pname != null then "${args.pname}-${args.version}" else null;