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 
'use strict';
const rollup = require('rollup').rollup;
const babel = require('rollup-plugin-babel');
const commonjs = require('rollup-plugin-commonjs');
const alias = require('rollup-plugin-alias');
const uglify = require('rollup-plugin-uglify');
const replace = require('rollup-plugin-replace');
const chalk = require('chalk');
const escapeStringRegexp = require('escape-string-regexp');
const join = require('path').join;
const resolve = require('path').resolve;
const fs = require('fs');
const rimraf = require('rimraf');
const argv = require('minimist')(process.argv.slice(2));
const Modules = require('./modules');
const Bundles = require('./bundles');
const propertyMangleWhitelist = require('./mangle').propertyMangleWhitelist;
const sizes = require('./plugins/sizes-plugin');