Summary:
The XMLHttpRequest jest tests were attempting to call a private method in XMLHttpRequestBase.js (denoted by an _ prefix).
JS doesn't actually have any language-level support for private methods, the use of _ prefix is just a convention. But to prevent casually calling private methods externally, we have a transform that mangles the names of prefixed methods so that that attempting to call them will fail.
Using a double _ bypasses this name-mangling mechanism, while still making it clear that the method is intended to be private.
Reviewed By: javache
Differential Revision: D3276261
fb-gh-sync-id: e0c17e1003d2df09d1a16f78ae9d95bef923d74e
fbshipit-source-id: e0c17e1003d2df09d1a16f78ae9d95bef923d74e
963a53b1a7