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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