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 
if (preserveVersionHeader && comment.pos === 0 && comment.col === 0) {
          
          if (
            headerSanityCheck &&
            comment.value.indexOf(headerSanityCheck) === -1
          ) {
            
            throw new Error(
              'Expected the first comment to be the file header but got: ' +
                comment.value
            );
          }
          return true;
        }
        return !removeComments;
      },
    },
    mangleProperties: mangle && manglePropertiesOnProd
      ? {
          ignore_quoted: true,
          regex: mangleRegex,
        }
      : false,