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 
replace(stripEnvVariables(bundleType === RN_PROD)),
        
        commonjs(getCommonJsConfig(bundleType)),
        uglify(
          uglifyConfig({
            mangle: false,
            manglePropertiesOnProd,
            preserveVersionHeader: true,
            removeComments: true,
            headerSanityCheck,
          })
        )
      );
      break;
  }
  
  plugins.push(
    sizes({
      getSize: (size, gzip) => {
        const key = `${filename} (${bundleType})`;
        Stats.currentBuildResults.bundleSizes[key] = {
          size,
          gzip,
        };
      },
    })
  );
  return plugins;
}
function createBundle(bundle, bundleType) {