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 shouldSkipBundleType = bundle.bundleTypes.indexOf(bundleType) === -1;
  if (shouldSkipBundleType) {
    return Promise.resolve();
  }
  if (requestedBundleTypes.length > 0) {
    const isAskingForDifferentType = requestedBundleTypes.every(
      requestedType => bundleType.indexOf(requestedType) === -1
    );
    if (isAskingForDifferentType) {
      return Promise.resolve();
    }
  }
  if (requestedBundleNames.length > 0) {
    const isAskingForDifferentNames = requestedBundleNames.every(
      requestedName => bundle.label.indexOf(requestedName) === -1
    );
    if (isAskingForDifferentNames) {
      return Promise.resolve();
    }
  }
  const filename = getFilename(bundle.name, bundle.hasteName, bundleType);
  const logKey =
    chalk.white.bold(filename) + chalk.dim(` (${bundleType.toLowerCase()})`);
  const format = getFormat(bundleType);