ES6/ES7 coding style at tools/build.js

I cannot use `import` (eg: import fs from 'fs';...) at this file.

---
Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/39351528-es6-es7-coding-style-at-tools-build-js?utm_campaign=plugin&utm_content=tracker%2F18782092&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F18782092&utm_medium=issues&utm_source=github).

Start by reading tools/build.js and identify why ES6/ES7 import syntax cannot be used there. Check the repository's existing build process, then verify that the file accepts the requested import style and that the build still completes successfully.

Written by the indexing model from the issue text.