Build script missing

[greenwombat]

• I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior

To build a local distribution, CONTRIBUTING page says:

From the root folder of the material-ui project
yarn build

Current Behavior

Build command is not found

Steps to Reproduce (for bugs)

yarn build
yarn run v1.3.2

error Command "build" not found.

info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.`

Context

I can not build a local version to test against before submitting a PR

Your Environment

Tech	Version
Material-UI	next