Fix polyfilling of regeneratorRuntime to avoid setting it to undefined in some situations

Summary:
This diff fixes an issue that caused the problem with `regeneratorRuntime` last Friday (more info: https://fb.facebook.com/groups/frontendsupport/permalink/2427883350560427/).

The root issue is that both `InitializeCore` and `FBInitializeCore` are included in the same bundle, this fix just prevents the bundle from being invalid once this happens..

*copied from: https://our.intern.facebook.com/intern/diff/D10444264/?transaction_id=595485237532887*

The way that `regeneratorRuntime` is polyfilled is not correct:

```
polyfillGlobal('regeneratorRuntime', () => {
  // The require just sets up the global, so make sure when we first
  // invoke it the global does not exist
  delete global.regeneratorRuntime;
  require('regenerator-runtime/runtime');
  return global.regeneratorRuntime;
});
```

Since a `require`d module is only evaluated once (no matter how many times it's required), defining (and calling) a getter for `global.regeneratorRuntime` twice will end up storing `undefined` to `global.regeneratorRuntime`.

There were no issues before this diff because the ordering of requires made things work by coincidence, e.g the following code will work:

```
// Set up regenerator for the first time
polyfillGlobal('regeneratorRuntime', () => {
  delete global.regeneratorRuntime;
  require('regenerator-runtime/runtime');
  return global.regeneratorRuntime;
});

// Set up regenerator for the second time. This will just override the previous getter (which has not even got executed so
// the `regenerator-runtime/runtime` module has not been evaluated yet.
polyfillGlobal('regeneratorRuntime', () => {
  // The require just sets up the global, so make sure when we first
  // invoke it the global does not exist
  delete global.regeneratorRuntime;
  require('regenerator-runtime/runtime');
  return global.regeneratorRuntime;
});

// Now access regenerator
global.regeneratorRuntime;
```

But the following code won't work:

```
// Set up regenerator for the first time
polyfillGlobal('regeneratorRuntime', () => {
  delete global.regeneratorRuntime;
  require('regenerator-runtime/runtime');
  return global.regeneratorRuntime;
});

// Access regenerator. This will cause the previous getter to be called, and the `regenerator-runtime/runtime` module will get evaluated.
// Here, `global.regeneratorRuntime` will have a correct value.
global.regeneratorRuntime;

// Set up regenerator for the second time. This will define a new getter for `global.regeneratorRuntime`, which will delete `delete global.regeneratorRuntime`
// and return undefined (note that `require('regenerator-runtime/runtime');` is a noop since the module has been already evaluated).
polyfillGlobal('regeneratorRuntime', () => {
  // The require just sets up the global, so make sure when we first
  // invoke it the global does not exist
  delete global.regeneratorRuntime;
  require('regenerator-runtime/runtime');
  return global.regeneratorRuntime;
});
```

Reviewed By: fromcelticpark

Differential Revision: D10483975

fbshipit-source-id: 5b3ef6e11c4fc4f79e3857c1ade9e7bc2beb6a39

Author: rafeca

Libraries/Core/InitializeCore.js

@@ -91,8 +91,10 @@ polyfillGlobal('regeneratorRuntime', () => {
   // The require just sets up the global, so make sure when we first
   // invoke it the global does not exist
   delete global.regeneratorRuntime;
-  require('regenerator-runtime/runtime');
-  return global.regeneratorRuntime;
+
+  // regenerator-runtime/runtime exports the regeneratorRuntime object, so we
+  // can return it safely.
+  return require('regenerator-runtime/runtime');
 });
 
 // Set up timers