How to use the @microsoft/web-library-build.serial function in @microsoft/web-library-build

shouldFlatten: false,
  copyTo: {
    [path.join(distFolder, 'sass')]: [
      'node_modules/office-ui-fabric-core/dist/sass/**/*.*'
    ],
    [path.join(distFolder, 'css')]: [
      'node_modules/office-ui-fabric-core/dist/css/**/*.*'
    ]
  }
});

// Short aliases for subtasks.
build.task('webpack', build.webpack);
build.task('tslint', build.tslint);
build.task('ts', build.typescript);
build.task('sass', build.serial(build.preCopy, build.sass));
build.task('karma', build.karma);
build.task('test', serial(build.sass, build.typescript, build.karma));
// build.task('visualtest', serial(build.sass, build.typescript, build.webpack, visualTestClean, visualTest));

// initialize tasks.
build.initialize(gulp);