…ible. (#8024)

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| Q                        | A <!--(Can use an emoji 👍) -->
| ------------------------ | ---
| Fixed Issues?            | Fixes #7118
| Patch: Bug Fix?          | Y
| Major: Breaking Change?  | N
| Minor: New Feature?      | N
| Tests Added + Pass?      | Yes
| Documentation PR Link    | 
| Any Dependency Changes?  |
| License                  | MIT

* Fix private property parsing in Flow

* Flow tests updated

* Fix type error

* Appropriate name was given to test folder

* Fix

* Empty

* Correct type annotation

* Add required changes in generator package

* Add required changes in flow-strip-types

…nge.

…ey are known to be available.

Only reference helpers from external/runtime helpers if they are known to be available.

* added test case

* remove unnessecary parameter

* check for type directory as well in readRecursive filter

In Babel 6, we had `@babel/runtime` which _always_ uses `core-js` in its helpers, meaning that anyone who wants to use our helpers in a centralized modular way had to also use core-js, which isn't always ideal.

Early in the Babel 7 alpha, we added a `useBuiltins: true` option that would use separate copies of the helpers that would skip loading `core-js`, but this _still_ requires users to have an indirect npm dependency on `core-js`.

This PR splits our setup into two separate runtimes:

* ```
  'plugins': [
    ['transform-runtime'],
  ]
  ```
  with `npm install --save @babel/runtime` that just includes our helpers, and relies on users to globally polyfill any APIs they need.
* ```
  'plugins': [
    ['transform-runtime', { corejs: 2 }],
  ]
  ```
  with `npm install --save @babel/runtime-corejs2` that includes everything `@babel/runtime` does, but also includes a passthrough API for `corejs@2.x` and rewrites all of the helper modules to reference `core-js`.