anchor.diff:        return filePath;
       }
       if (import.startsWith(PACKAGE_PREFIX)) {
  +      if (_env["pkgRoot"] == null) {
  +        // No package-root given, do not resolve package: URIs.
  +        return null;
  +      }
         var filePath =
             "${_env["pkgRoot"]}"
             "/${import.substring(PACKAGE_PREFIX.length, import.length)}"; 
  
positive.code:   parser.addOption("sdk-root", abbr: "s",
                     help: "path to the SDK root");
    parser.addOption("package-root", abbr: "p",
                     help: "path to the package root",
                     defaultsTo: ".");
    parser.addOption("in", abbr: "i",
                     help: "input(s): may be file or directory");
    parser.addOption("out", abbr: "o",
  
negative.code:     exit(0);
    }
  
    env.sdkRoot = args["sdk-root"];
    if (env.sdkRoot == null) {
      if (Platform.environment.containsKey("SDK_ROOT")) {
        env.sdkRoot =
          join(absolute(normalize(Platform.environment["SDK_ROOT"])), "lib");
      }
    } else {
      env.sdkRoot = join(absolute(normalize(env.sdkRoot)), "lib");
    }
    if ((env.sdkRoot != null) && !FileSystemEntity.isDirectorySync(env.sdkRoot)) {
      fail("Provided SDK root '${args["sdk-root"]}' is not a valid SDK "
           "top-level directory");
    }
  
    env.pkgRoot = args["package-root"];
    if (env.pkgRoot != null) {
      env.pkgRoot = absolute(normalize(args["package-root"]));
      if (!FileSystemEntity.isDirectorySync(env.pkgRoot)) {
        fail("Provided package root '${args["package-root"]}' is not directory.");
  