Problem

Currently, process.env.NODE_ENV is set to be production in the next/jest environment. I think the behavior isn't desirable — there are legit cases where we need to assert on DEV-only warnings in tests (for instance, make sure React doesn't fire unique "key" warnings).

Solution

This PR changes the behavior: source code gets compiled in development mode such that React and friends don't omit dev mode warnings. The actual NODE_ENV value is left blank. By default, Jest will set NODE_ENV to "test" (see jestjs/jest@3a38ddf). You can override the behavior via explicitly setting the NODE_ENV environment variable.

Test Plan

• run yarn test-unit: process.env.NODE_ENV === "test"
• run NODE_ENV=development yarn test-unit: process.env.NODE_ENV === "development"

Bug

• Related issues linked using fixes #number
• Integration tests added
• Errors have helpful link attached, see contributing.md

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.
• Errors have helpful link attached, see contributing.md

Documentation / Examples

• Make sure the linting passes by running yarn lint