[Dev workflow] Add hook to run Flow type checks on npm run build

[gino-m]

Described in facebook/create-react-app#5995 (comment)

[bugzpodder]

This is the only project in google org that uses flow? I thought google is big on typescript? Is there any reason why flow is picked over ts?

[gino-m]

When I looked into it, it seemed integrating ts with React was non-trivial, and I wanted to minimize the number of custom integrations to reduce complexity. This may have changed since, but since this is a 20%/best-effort project, I haven't had time to investigate this fully.

Also, Ground uses Firebase for db triggers and hosted functions, and the examples in the docs are primarily ES6, so I preferred to use one language across both web client and server-side. ts integration is available, but again, unfortunately I haven't had time to try it out.

[gino-m]

Obsolete; will be reimplemented in new version.