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 packageName = Packaging.getPackageName(bundle.name);
  console.log(`${chalk.bgYellow.black(' BUILDING ')} ${logKey}`);
  return rollup({
    entry: bundleType === FB_DEV || bundleType === FB_PROD
      ? bundle.fbEntry
      : bundle.entry,
    external: Modules.getExternalModules(
      bundle.externals,
      bundleType,
      bundle.isRenderer
    ),
    onwarn: handleRollupWarnings,
    plugins: getPlugins(
      bundle.entry,
      bundle.babelOpts,
      bundle.paths,
      filename,
      bundleType,
      bundle.hasteName,
      bundle.isRenderer,
      bundle.manglePropertiesOnProd,
      bundle.useFiber,
      bundle.modulesToStub
    ),
  })
    .then(result =>
      result.write(
        updateBundleConfig(
          bundle.config,
          filename,