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 
Stats.saveResults();
      if (argv['extract-errors']) {
        console.warn(
          '\nWarning: this build was created with --extract-errors enabled.\n' +
            'this will result in extremely slow builds and should only be\n' +
            'used when the error map needs to be rebuilt.\n'
        );
      }
    })
    .catch(err => {
      console.error(err);
      process.exit(1);
    });
});
function runWaterfall(promiseFactories) {
  if (promiseFactories.length === 0) {
    return Promise.resolve();
  }
  const head = promiseFactories[0];
  const tail = promiseFactories.slice(1);
  const nextPromiseFactory = head;
  const nextPromise = nextPromiseFactory();
  if (!nextPromise || typeof nextPromise.then !== 'function') {
    throw new Error('runWaterfall() received something that is not a Promise.');
  }
  return nextPromise.then(() => {
    return runWaterfall(tail);
  });
}