You could make the argument that this should be optional, but it doesn't
work without it so we might as well just enforce it.
Makes `jest` work by default.
Turns out jest is _incredibly_ slow at resolving require paths like `require('fbjs/lib/foo')`. Like several milliseconds per require. Really adds up when all our files require `invariant` and `warning`. Here's a temporary hack to make things fast again.
Test Plan:
```
npm test src/renderers/shared/reconciler/__tests__/ReactCompositeComponent-test.js
```
has a self-proclaimed runtime of ~8 seconds now instead of ~35 seconds.
This reworks a few things in building and distributing React. The biggest change is using fbjs to share dependencies with other libraries. We're also using Gulp for some build steps.
