Error with webpack

Hi guys,

I am getting following error with latest git version:
Done in 1.75s.
yarn run v0.21.3
$ yarn run webpack -- --config webpack.config.js
yarn run v0.21.3
$ "/home/zangetsu/devel/tmp/pgadmin4/web/node_modules/.bin/webpack"
--config webpack.config.js
module.js:491
throw err;
^

Error: Cannot find module 'optimize-css-assets-webpack-plugin'
at Function.Module._resolveFilename (module.js:489:15)
at Function.Module._load (module.js:439:25)
at Module.require (module.js:517:17)
at require (internal/module.js:11:18)
at Object.<anonymous>
(/home/zangetsu/devel/tmp/pgadmin4/web/webpack.config.js:11:33)
at Module._compile (module.js:573:30)
at Object.Module._extensions..js (module.js:584:10)
at Module.load (module.js:507:32)
at tryModuleLoad (module.js:470:12)
at Function.Module._load (module.js:462:3)
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this
command.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this
command.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this
command.

Commit:
commit c4393a7795eef18b311724f45b6c9c0dae01926d (HEAD -> master,
origin/master, origin/HEAD)
Author: Surinder Kumar <surinder(dot)kumar(at)enterprisedb(dot)com>
Date: Thu Aug 31 14:50:22 2017 +0100