title: In TypeScript, what's the difference between the `lib.es6.d.ts` and `lib.es2015.d.ts`?
body: <p>From my understanding, the ES6 and the ES2015 is the same thing. But in typescript, there are 2 type declaration for these 2 things: <code>lib.es6.d.ts</code> and <code>lib.es2015.d.ts</code>. What's difference? Which one should I use?</p>