Summary: @public
jest is running the polyfill multiple times on the same environment (cc @cpojer, need to fix that!). By default jest doesn't have XMLHttpRequest polyfilled so it'll define a property with writable to be false. It'll fatal the second time it tries to override XMLHttpRequest.
The hacky workaround is to make properties that do not exist with writable: true. But the long term fix would be to make jest stop running the polyfill multiple times.
Reviewed By: @javache
Differential Revision: D2532019
fb-gh-sync-id: a82abf69541781a64a0744798c736f90833e28cb