Test Plan: Changed the preprocessor to log the output of babel.transform and saw
```
var _jsxFileName = 'src/isomorphic/modern/element/__tests__/ReactJSXElementValidator-test.js';
```
in the resulting output, instead of an absolute path.
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.
