Check whether a source string looks like an ES6 module.
This doesn't actually execute the code,
and doesn't check other module types.
So source strings without any module loaders returns false.
This is just what I need from https://github.com/yahoo/js-module-formats, which actually executes the sauce string in a subcontext.
var isES6Module = require('is-module');
console.log(isES6Module('import * from "emitter";')) // => true