Use babel runtime instead of relying on global babelHelpers and regenerator (#198)

Summary:
**Summary**

The RN transformer currently relies on the enviroment providing babelHelpers and regeneratorRuntime as globals by using 'babel-external-helpers'. This wasn't really a problem before since helpers were stable and we could maintain our copy easily but it seems like there are more now with babel 7 and it makes sense to include only those used by the app.

This is exactly what babel/transform-runtime does. It will alias all helpers and calls to regeneratorRuntime to files in the babel/runtime package.

This will solve issues like this https://github.com/facebook/react-native/issues/20150 caused by missing babelHelpers. This solution also avoids bloating babelHelpers to fix OSS issues like the one linked before.

**Test plan**

- Updated tests so they all pass.
- Tested that it actually works by applying the changes locally in an RN app.
- Added a test for async functions, to make sure regenerator is aliased properly and doesn't depend on the global.
- Made sure require-test.js still fails if the require implementation contains babel helpers (by adding an empty class in the file).
Pull Request resolved: https://github.com/facebook/metro/pull/198

Reviewed By: mjesun

Differential Revision: D8833903

Pulled By: rafeca

fbshipit-source-id: 7081f769f288ab358ba89ae8ee72a513bb12e225

jest/preprocessor.js

@@ -53,6 +53,7 @@ module.exports = {
       options: {
         ast: true, // needed for open source (?) https://github.com/facebook/react-native/commit/f8d6b97140cffe8d18b2558f94570c8d1b410d5c#r28647044
         dev: true,
+        disableBabelRuntime: true,
         inlineRequires: true,
         minify: false,
         platform: '',