Use ES6 modules natively via Polyfill

Use ES6 modules natively via Polyfill

This commit introduces the "Browser ES Module Loader" polyfill
to support developing with native ES6 modules, without any compilation
step (files are passed through Babel in the browser).  This should not
be used in production -- a pre-compiled version passed through babel
ahead of time (as produced by the `npm install` hook or
`utils/use_require.js`) should be used instead.