Allow builtin detection for ES6

[guybedford]

import {readFile} from 'fs' should ideally work without fs needing to be an explicit dependency.

Similarly for the Buffer and process global shims, and resolution system.

The assumption is that the above is what ES6 will look like in Node eventually anyway.

[guybedford]

This is making assumptions about how Node itself will handle ES6, which is not our place to do.