`if (process.CI)` check looks like a typo, here
https://github.com/facebook/react/blob/master/scripts/tasks/eslint.js#L13

May be I'm wrong but I haven't found what `process.CI` is.
If CI is https://circleci.com/docs/2.0/env-vars/#circleci-environment-variable-descriptions it should be checked as `process.env.CI`

/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
'use strict';
const runESLint = require('../eslint');
console.log('Linting all files...');
if (!process.CI) {
  console.log('Hint: run `yarn linc` to only lint changed files.');
}
if (runESLint({onlyChanged: false})) {
  console.log('Lint passed.');
} else {
  console.log('Lint failed.');
  process.exit(1);
}