Our built files are missing license headers. 

cjs has no headers:
https://unpkg.com/react@16.0.0-beta.5/cjs/react.development.js
https://unpkg.com/react-dom@16.0.0-beta.5/cjs/react-dom.development.js

umd has headers identifying the bundle name but not the license:
https://unpkg.com/react@16.0.0-beta.5/umd/react.development.js
https://unpkg.com/react-dom@16.0.0-beta.5/umd/react-dom.development.js

Both should have license headers similar to this:

```
/**
 * React v15.6.1
 *
 * Copyright 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 */
```

We likely just need to change the code in getBanner in scripts/rollup/build.js. https://github.com/facebook/react/blob/b4a3e3b34594759c90aef1694d2166b2d1126fa1/scripts/rollup/build.js#L78

You can test by running `yarn build` and looking in build/packages.

cc @trueadm 
const Stats = require('./stats');
const syncReactDom = require('./sync').syncReactDom;
const syncReactNative = require('./sync').syncReactNative;
const Packaging = require('./packaging');
const Header = require('./header');
const closure = require('rollup-plugin-closure-compiler-js');
const UMD_DEV = Bundles.bundleTypes.UMD_DEV;
const UMD_PROD = Bundles.bundleTypes.UMD_PROD;
const NODE_DEV = Bundles.bundleTypes.NODE_DEV;
const NODE_PROD = Bundles.bundleTypes.NODE_PROD;
const FB_DEV = Bundles.bundleTypes.FB_DEV;
const FB_PROD = Bundles.bundleTypes.FB_PROD;
const RN_DEV = Bundles.bundleTypes.RN_DEV;
const RN_PROD = Bundles.bundleTypes.RN_PROD;
const reactVersion = require('../../package.json').version;
const requestedBundleTypes = (argv.type || '')
  .split(',')
  .map(type => type.toUpperCase());
const requestedBundleNames = (argv._[0] || '')
  .split(',')
  .map(type => type.toLowerCase());
const syncFbsource = argv['sync-fbsource'];
const syncWww = argv['sync-www'];
const mangleRegex = new RegExp(
  `^(?${propertyMangleWhitelist
    .map(prop => `!${escapeStringRegexp(prop)}`)
    .join('|')}$).*$`,
  'g'
);
function getHeaderSanityCheck(bundleType, hasteName) {